if (Grade::exists($id, $student_id)) {
     $newgrade = Grade::get_by_subj_stud($id, $student_id);
     $newgrade->delete();
 }
 $newgrade = new Grade();
 $newgrade->subject_id = $id;
 $newgrade->student_id = $student_id;
 $newgrade->grade = 0;
 $newgrade->create();
 $pre_subject = Subject::get_by_id($id);
 if (!EvaluatedSubject::exists($id, $student_id)) {
     $grade = Grade::get_by_subj_stud($pre_subject->id, $student_id);
     $gradepre = Grade::get_by_subj_stud($pre_subject->prereq_subject_id, $student_id);
     if (has_prerequisite($pre_subject->prereq_subject_id, $student_subjects)) {
         if ($gradepre != null) {
             if (Grade::get_by_subj_stud($pre_subject->prereq_subject_id, $student_id)->grade <= 74) {
                 $warnings .= "Cannot add " . $pre_subject->code . " because it's previous subject:" . Subject::get_by_id($pre_subject->prereq_subject_id)->code . " is failed\n\n";
             } else {
                 $eval_subject = new EvaluatedSubject();
                 $eval_subject->student_id = $student_id;
                 $eval_subject->subject_id = $id;
                 $eval_subject->create();
             }
         }
     } else {
         $eval_subject = new EvaluatedSubject();
         $eval_subject->student_id = $student_id;
         $eval_subject->subject_id = $id;
         $eval_subject->create();
     }
 }
<?php

require_once "../includes/initialize.php";
$student_id = $_GET['student_id'];
$subject_ids2 = $_POST['subject_ids2'];
$message = "success";
foreach ($subject_ids2 as $id) {
    $subject_id = EvaluatedSubject::get_by_id($id)->subject_id;
    $newgrade = Grade::get_by_subj_stud($subject_id, $student_id);
    if ($newgrade != null) {
        $newgrade->delete();
    }
    $object = EvaluatedSubject::get_by_id($id);
    if ($object != null) {
        $object->delete();
    }
}
echo $message;