I have a trigger and when running hit the below error. not sure how to fix this, requesting expert help.

Error:Apex trigger updateReliefTeacheronTeacherClassSession caused an unexpected exception, contact your administrator: updateReliefTeacheronTeacherClassSession: execution of AfterUpdate caused by: System.QueryException: Non-selective query against large object type (more than 200000 rows). Consider an indexed filter or contact salesforce.com about custom indexing. Even if a field is indexed a filter might still not be selective when: 1. The filter value includes null (for instance binding with a list that contains null) 2. Data skew exists whereby the number of matching rows is very large (for instance, filtering for a particular foreign key value that occurs many times): Trigger.updateReliefTeacheronTeacherClassSession: line 14, column 1

trigger updateReliefTeacheronTeacherClassSession on Staff_Leave__c (after insert, after update)
    {
        map<string, Staff_Leave__c> ObjMap = new map<string, Staff_Leave__c>();
        DateTime myDateTime;
        String dayOfWeek;
        
  
        for(Staff_Leave__c obj: Trigger.new)
        {   List<Class__c> classList = [SELECT Id,Day__c from Class__c where id =: obj.Active_Class__c];
            
            if (obj.Relief_Teacher__c != Null)
            {   myDateTime = (DateTime)obj.date_of_leave__c;
                dayOfWeek = myDateTime.format('E');
                
                for(Class__c classRec:classList)
                {
                    if(dayOfWeek != classRec.day__c)
                    {
                       obj.addError('Please check leave dates.'); 
                    }
                    else
                    {
                        ObjMap.put(obj.Teacher__c, obj);
                    }
                }
                System.debug('-----Teacher is = '+ obj.Teacher__c + ' ------');
            }
        }
         
        List<Teacher_Class_Session__c> T = [SELECT Id,Teacher__c, Staff_Leave__c ,Assigned_Teacher__c,Relief_Type__c , Class__c,Class__r.teacher__c, Session_Date__c, Attendant_Status__c FROM Teacher_Class_Session__c WHERE Class__r.teacher__c IN :ObjMap.KeySet()];
        List<Teacher_Class_Session__c> TUpdateList = new List<Teacher_Class_Session__c>();
         
        for(Teacher_Class_Session__c c: T)
        {   System.debug('----------- Iterating through all class sessions-----');
            
            Staff_Leave__c obj = ObjMap.get(c.class__r.teacher__c);
            System.debug('Class session id = '+ c.id);
            System.debug('c.teacher= ' + c.class__r.teacher__c);
            System.debug('obj teacher= ' +obj.teacher__c);
            System.debug('c.class= ' + c.class__c + ' ' + 'obj class= ' +obj.Active_Class__c );
            System.debug('c.date= ' + c.session_date__c + ' ' + 'obj class= ' +obj.date_of_leave__c);
                        
            if(obj.Teacher__c == c.Class__r.teacher__c && obj.Active_Class__c == c.Class__c && c.session_date__c == obj.date_of_leave__c)
            { System.debug('Match found');
             c.Teacher__c = obj.Relief_Teacher__c;
             c.Relief_Type__c = obj.Leave_Type__c;
             c.Attendant_Status__c= 'Relief';
             c.Staff_Leave__c = obj.id;
             tUpdateList.add(c);
             }
         
           
        }
     
        if(tUpdateList.size() > 0)
        {
            update tUpdateList;
        }
        
    }