<?php

    $array = [
        '1',
        '50419104',
        'ADAM RIZQI ROMADHON',
        '1IA10',
        '2IA02',
        '2',
        '50419315',
        'AHMAD FADLI ATTHORIQ',
        '1IA10',
        '2IA13',
        '3',
        '50419356',
        'AIDA HANUM',
        '1IA10',
        '2IA05',
        '4',
        '50419666',
        'AMMAR RIDWAN DARMA',
        '1IA10',
        '2IA01',
        '5',
        '51419027',
        'ARIF SETIAWAN',
        '1IA10',
        '2IA08',
        '6',
        '51419275',
        'BAGUS KUSNANTO',
        '1IA10',
        '2IA08',
        '7',
        '56419982',
        'BAGUS SAPUTRA',
        '1IA10',
        '2IA09',
        '8',
        '51419427',
        'CAROLINE SHERINA HANJANI',
        '1IA10',
        '2IA06',
        '9',
        '51419446',
        'CHANDRA RAFLI ARDIYANTO',
        '1IA10',
        '2IA10',
        '10',
        '51419788',
        'DIDIN NUR YAHYA',
        '1IA10',
        '2IA07',
    ];

    $data_siswa = array_chunk($array, 5);
    $kelas_baru = [];
    foreach($data_siswa as $siswa){
        $kelas_baru[$siswa[4]][] = $siswa;
    }

    print_r($kelas_baru);