Dentist's excuse slip.

There is no reason to cover up for him.  A single unexcused tardy is no big deal. The school office is doing its job to check on him.  Even if you cover for him, without his dentist's excuse slip, his tardiness could not be excused.  Will the teacher take his word for it? Second, dentist appointment is an appointment, as such, parents should notify   the schoo l when the appoint becomes unreasonably long.   It is a lesson learned, much more important than GPA. Who knows if he confesses to the teacher, the teacher may forgive the penalty.