Već sam bio napisao, u upitu
$query = "INSERT INTO studenti (ime, prezime, br_indexa, stud_prog, email, sifra, mentor, god_studija) VALUES
('$student->ime','$student->prezime','$student->br_indexa','$student->Stud_prog',
'$student->email','$student->sifra','$student->mentor','$student->god_studija')";
Umesto $student napiši $this
Zatim u
index.php imaš
$student -> br_index=$_POST['form_brindex'];
u klasi studenti
public $br_ind;
a u upitu
$student->br_indexa