Unless I select any options on tab "Student", i could'nt search any on Tab "Attendence"
Why is this happening?
Visualforce page:

<apex:page standardController="Teacher__c" action="{!fetchFailingPage}" extensions="RGMTeacherController" sidebar="false" showHeader="false">
    <style>
        .activeTab {background-color: #236FBD; color:white; background-image:none}
        .inactiveTab { background-color: lightgrey; color:black; background-image:none}
    </style>
          
      <apex:form id="loginForm" forceSSL="true">
       
            <apex:tabPanel switchType="client" tabClass="activeTab" inactiveTabClass="inactiveTab" selectedTab="Information">
                
                <apex:tab id="home" label="Home" title="Home">                   
                </apex:tab>
                
                <apex:tab label="Information" name="Information" id="InformationTab" >                                         
                </apex:tab>

                <apex:tab id="student" label="Student" title="Student">
                    <apex:pageBlock >
                        <apex:selectList label="Standard" id="choosestandard" required="true"  alue="{!standard}" >
                            <apex:selectOptions value="{!standarditems}" id="standarditem" />                           
                        </apex:selectList> 
                        <apex:selectList label="Section" id="choosesection" required="true" value="{!section}" >
                            <apex:selectOptions value="{!sectionitems}" id="sectionitem" />                           
                        </apex:selectList>
                        <apex:selectList label="Subject" id="choosesubject" required="true" value="{!subject}" >
                            <apex:selectOptions value="{!subjectitems}" id="subjectitem" />                           
                        </apex:selectList>
                        <apex:commandButton action="{!studentsearch}" value="Search" reRender="stulistid" />               
                    
                    
                        <apex:outputPanel id="stulistid" >
                        <apex:pageblocktable value="{!studentlist}" var="a" id="pbTable" >
                            <apex:column value="{!a.Name}"/>
                            <apex:column value="{!a.Roll_Number__c}"/>
                            <apex:column value="{!sfUrl}{!a.Id}"/>
                            <apex:column ><apex:outputLink value="{!URLFOR($Action.Student__c.View, a.id)}">{!a.name}</apex:outputLink></apex:column>
                        </apex:pageblocktable>
                        </apex:outputPanel>
                    </apex:pageBlock>
                </apex:tab>
                
                <apex:tab id="event" label="Events" title="Events">              
                </apex:tab>
                
                <apex:tab id="attendence" label="Attendence" title="Attendence">              
                    
                    <apex:pageBlock title="Make Attendence">                       
                            <apex:pageBlockSection >
                                <apex:selectList label="Standard" id="stustandard" value="{!stanatt}" >
                                    <apex:selectOption itemValue="L.K.G" itemLabel="L.K.G"/>
                                    <apex:selectOption itemValue="U.K.G" itemLabel="U.K.G"/>
                                    <apex:selectOption itemValue="I" itemLabel="I"/>
                                    <apex:selectOption itemValue="II" itemLabel="II"/>                        
                                </apex:selectList>
                                <apex:selectList label="Section" id="stusec" value="{!secatt}" >
                                    <apex:selectOption itemValue="A" itemLabel="A"/>
                                    <apex:selectOption itemValue="B" itemLabel="B"/>                       
                                </apex:selectList>
                            </apex:pageBlockSection>
                            <apex:pageBlockButtons location="Top">
                            <apex:commandButton action="{!makatt}" value="Search" reRender="xyz"/>
                        </apex:pageBlockButtons>
                    </apex:pageBlock>
                    <apex:outputPanel id="xyz">
                    <apex:pageBlock title="Attendence List">                   
                        <apex:pageBlockTable value="{!studentattlist}" var="itr">
                            <apex:column value="{!itr.Name}"/>
                            <apex:column value="{!itr.Roll_Number__c}"/>
                        </apex:pageBlockTable>
                        
                        <apex:pageBlockButtons >
                            <apex:commandButton action="{!saveatt}" value="Save" />
                        </apex:pageBlockButtons>
                    </apex:pageBlock>
                    </apex:outputPanel>              
                </apex:tab>
                
            </apex:tabPanel>
      </apex:form>
       
</apex:page>


Controller:

public class RGMTeacherController {
    public string username {get;set;}
    public string password {get;set;}
    public Boolean showRecords{get;set;}
    public Boolean attbool{get;set;}
    public List<Student__c> studentlist{get;set;}
    public List<Student__c> studentattlist{get;set;}
    public string standard{get;set;}
    public string section{get;set;}
    public string subject{get;set;}
    public string stanatt{get;set;}
    public string secatt{get;set;}
    public string[] sections;
    public string[] standards;
    public string[] subjects;
    public String sfUrl {get;set;}
    public String failingPageResponse { get; set; }

     
    public RGMTeacherController(ApexPages.StandardController controller) {
        attbool = false;
        sfUrl = 'https://c.na34.salesforce.com/';
        Teacher__c TeacherId = [SELECT Id, Name, Email__c, Section__c, Standard__c, Subject__c, Contact_Number__c FROM Teacher__c WHERE Username__c =: username AND Password__c =: password LIMIT 1];
        ApexPages.currentPage().getParameters().put('id', TeacherId.Id);
        sections = TeacherId.Section__c.split(';', 7);
        standards = TeacherId.Standard__c.split(';', 7);
        subjects = TeacherId.Subject__c.split(';', 7);
    }
    public List<SelectOption> getstandarditems() {
        List<SelectOption> standardoptions = new List<SelectOption>();
         for(Integer i=0; i < standards.size(); i++) {
             standardoptions.add(new SelectOption(standards[i], standards[i]));
         }
        return standardoptions;
    }
    
    public String getstandard() {
        return standard;
    }
            
    public void setstandard(String standard) {
        this.standard = standard;
    }
    
    public void fetchFailingPage() {
       try {
           // Make a call to failing sites page here
           failingPageResponse = Page.RGM_Teachers_Page.getContent().toString();
       } catch (Exception e) {
           failingPageResponse = e.getTypeName() + ' : ' + e.getMessage() ;
       }       
    }   
    
    public List<SelectOption> getsectionitems() {
        List<SelectOption> sectionoptions = new List<SelectOption>();
        for(Integer i=0; i < standards.size(); i++) {
             sectionoptions.add(new SelectOption(sections[i], sections[i]));
         }
        return sectionoptions;
    }
    
    public String getsection() {
        return section;
    }
            
    public void setsection(String section) {
        this.section = section;
    }
    
    public List<SelectOption> getsubjectitems() {
        List<SelectOption> subjectoptions = new List<SelectOption>();
        for(Integer i=0; i < subjects.size(); i++) {
             subjectoptions.add(new SelectOption(subjects[i], subjects[i]));
         }
        return subjectoptions;
    }
    
    public String getsubject() {
        return subject;
    }
            
    public void setsubject(String subject) {
        this.subject = subject;
    }
    
    public String getsfurll() {
        return sfURL;      
    }

    public void studentsearch() {
        system.debug('*********************************'+standard);
        studentlist = [SELECT Id, Name, Roll_Number__c FROM Student__c WHERE Standard__c =: standard AND Section__c =: section];
        system.debug('*********************************'+studentlist);
        
    } 
    
    public void makatt() {
        system.debug('***************96******************');
        studentattlist = [SELECT Id, Name, Roll_Number__c FROM Student__c WHERE Standard__c =: stanatt AND Section__c =: secatt];
        system.debug('*********************************'+studentattlist);
        attbool = true;    
    }
    
    public void saveatt() {
              
    }   
    
}