Teacher tardiness = work without pay

Hi, I'm a university professor. Our school has implemented a policy that after 15 minutes late the teacher is marked "Absent without Notification". This means we don't get paid for the said class regardless if we taught the class even for 3 or 4 straight hours. Sometimes the attendance checker would see us go into the class but we would still be marked absent because we came in at the 16th or 17th minute after the start of class. On top of having no pay, they also put deductions on our performance evaluation.

One time we asked the dean if we can skip the class if we're already late for 15 minutes but we were told that we should still report to that class as much as possible - even without pay. This is really driving me crazy.

Is this practice legal under our labor code?

there are special rules for teachers, but imo, that is too harsh.

15m is good enough actually in most places 7mins is all you get!