How to write to get record without using query inside for loop in apex trigger ?

@Usman Ali @EMEA MVP Office Hours  

Bhavin Patel (Salesforce)

how to change the below code with best practice ?
 for(Id  c : pId)
        {
            List<Student__C> child = [select Department__c, isApproved__c, Name, Id FROM Student__c where Department__c =:c AND isApproved__C = True];
            pchild.put(c,child);
        }

trigger StudentTrigger on Student__c (after insert,after update,after delete,after undelete) 
{   
    if(trigger.isAfter &&  (trigger.isInsert || trigger.isUpdate || trigger.isUndelete))
    {      
        StudentTriggerHandler.getStudent(Trigger.newMap);
    }
    if(trigger.isDelete)
    {
        StudentTriggerHandler.getStudent(trigger.oldMap);
    }
}
///Handler
public class StudentTriggerHandler 
{
    public static void getStudent(Map<Id,Student__C> obj)
    { 
        set<ID> pId = new set<ID>();
        for(Student__C a : obj.values())
        {
            pId.add(a.Department__c);
        }
        Map<Id,List<Student__C>> pchild = new  Map<Id,List<Student__C>>();
        for(Id  c : pId)
        {
            List<Student__C> child = [select Department__c, isApproved__c, Name, Id FROM Student__c where Department__c =:c AND isApproved__C = True];
            pchild.put(c,child);
        }
        system.debug('pchild - ' + pchild);
        List<sobject> updatedstudent = new List<sobject>();
        for(Id idValue : pchild.keyset())
        {
            integer count =0 ;
            for( Student__C s : pchild.get(idValue ))
            {
                system.debug(' Map keyset Id Value' + idValue );
                system.debug(' list values : '+ s);
                count =count +1;
            }
            if (count == 0)
            {
                Department__c sc = new Department__c();
                sc.Id =idValue;
                sc.Child_Approved_Status__c ='';
                updatedstudent.add(sc);
            }
            else if(count == 1 )
            {
                Department__c sc = new Department__c();
                sc.Id =idValue;
                sc.Child_Approved_Status__c ='1 child approved';
                updatedstudent.add(sc);                
            }
            else if(count == 2 )
            {
                Department__c sc = new Department__c();
                sc.Id =idValue;
                sc.Child_Approved_Status__c ='2 children approved';
                updatedstudent.add(sc);
            }
            else if(count > 2 )
            {
                Department__c sc = new Department__c();
                sc.Id =idValue;
                sc.Child_Approved_Status__c ='All child approved';
                updatedstudent.add(sc);
            }
        }
        update updatedstudent;
    }
}