I have lookup relationship between 2 objects on both objects if i delete the parent record i want to delete the child record also .how can i do this.
I was trying to do this but it is not getting deleted can anyone help where I am doing wrong.can anyone help

trigger TriggerOnTeacher on Teacher__c (after insert, after update, before delete) {
    List<Student__c> studentRecordsToUpdate = new List<Student__c>();
    List<Student__c> studentRecordsToDelete = new List<Student__c>();
    Set<Id> studentIdsToDelete = new Set<Id>();
    if (Trigger.isUpdate) {
        for (Teacher__c teacherRecord : Trigger.new) {
            Student__c studentRecord = [SELECT Id, Name, Firstname__c, Lastname__c, Email__c, Mobile__c, DOB__c, Address__c, City__c, Country__c
                                        FROM Student__c
                                        WHERE Teacher__c = :teacherRecord.Id];
            if (studentRecord != null) {
                studentRecord.Name = teacherRecord.Name;
                studentRecord.Firstname__c = teacherRecord.Firstname__c;
                studentRecord.Lastname__c = teacherRecord.Lastname__c;
                studentRecord.Email__c = teacherRecord.Email__c;
                studentRecord.Mobile__c = teacherRecord.Mobile__c;
                studentRecord.DOB__c = teacherRecord.DOB__c;
                studentRecord.Address__c = teacherRecord.Address__c;
                studentRecord.City__c = teacherRecord.City__c;
                studentRecord.Country__c = teacherRecord.Country__c;
                studentRecordsToUpdate.add(studentRecord);
            }
        }
    }
    
    if (Trigger.isInsert) {
        for (Teacher__c teacherRecord : Trigger.new) {
            if(triggerCount.runonce()){
                Student__c studentRecord = new Student__c();
                studentRecord.Teacher__c = teacherRecord.Id;
                studentRecord.Name = teacherRecord.Name;
                studentRecord.Firstname__c = teacherRecord.Firstname__c;
                studentRecord.Lastname__c = teacherRecord.Lastname__c;
                studentRecord.Email__c = teacherRecord.Email__c;
                studentRecord.Mobile__c = teacherRecord.Mobile__c;
                studentRecord.DOB__c = teacherRecord.DOB__c;
                studentRecord.Address__c = teacherRecord.Address__c;
                studentRecord.City__c = teacherRecord.City__c;
                studentRecord.Country__c = teacherRecord.Country__c;
                studentRecordsToUpdate.add(studentRecord);
            }
        }
    }
    
    if (Trigger.isDelete) {
        for (Teacher__c teacherRecord : Trigger.old) {
            studentIdsToDelete.add(teacherRecord.Student__c);
        }
    }
    
    if (!studentRecordsToUpdate.isEmpty()) {
        upsert studentRecordsToUpdate;
    }
    
    if (!studentRecordsToDelete.isEmpty()) {
        List<Student__c> studentsToDelete = [SELECT Id FROM Student__c WHERE Teacher__c IN :studentIdsToDelete];
        delete studentsToDelete;
    }
}

Thanks,