Hello Casper,

Try the following code and let me know if it helps or not.

add_filter( 'bp_core_fetch_avatar_no_grav', function ( $no_grav, $params ) {

	if ( 'user' !== $params['object'] ) {
		return $no_grav;
	}

	$user_member_types = (array) bp_get_member_type( $params['item_id'], false );
        
        // Replace with your member types.
	$mtypes_with_default_avatars = array( 'student', 'teacher' );

	if ( array_intersect( $mtypes_with_default_avatars, $user_member_types ) ) {
		return true;
	}

	return false;
}, 15, 2 );

Please give it a try.

Regards
Ravi