That is basically what my plugin does, except that it includes all user types and all field types.
If you’re only doing this for one field and one user type I don’t even think you need the setting.
add_filter('acf/prepare_field/name=inscription_classe', 'my_admin_only_prepare_field');
function my_admin_only_prepare_field( $field ) {
// return false if is not admin (removes field)
if( !current_user_can('administrator') ) return false;
return $field;
}