$meta_query = array(
  array(
    array(
      'relation' => 'OR',
      array(
        'key' => '_accepted_teacher',
        'compare'    => 'NOT EXISTS',
      ),
      array(
        'key' => '_accepted_teacher',
        'value' => $user_id,
        'type'    => 'numeric',
        'compare'    => '=',
      ),
    ),
  ),
  array(
    'relation' => 'OR',
    array(
      'key' => '_prefered_teacher',
      'compare'    => 'NOT EXISTS',
    ),
    array(
      'key' => '_prefered_teacher',
      'value' => $user_id,
      'type'    => 'numeric',
      'compare'    => '=',
    ),
  ),
  array(
    'relation' => 'OR',
    array(
      array(
        'key' => 'student_id',
        'value' => $student_ids,
        'type'    => 'numeric',
        'compare'    => 'IN',
      ),
      array(
        'key' => '_date_available',
        'value'   => $end_time,
        'type'    => 'DATETIME',
        'compare'    => '<',
      ),
    ),
    array(
      'key' => 'student_id',
      'value' => $student_ids,
      'type'    => 'numeric',
      'compare'    => 'NOT IN',
    ),
  ),
);