Punishment suggestion

Failing 3 subjects at your exams is very serious, I must say. So it will be harsh.

He should use a strap or a belt if he doesn't have a strap on your bottom 30 times, in diaper position (laying on your back with your legs up). But that's not all. You should then spread your legs wide apart for the belt where it'll hurt even more. Right there between your legs. 20 times.

Then you'll have your corner time after, he can position you how he likes but I think standing hands behind your head and legs spread, alternating between facing the wall and facing out towards the room is a good idea. 30 minutes.

All this while completely naked.

Three subjects failed doesn't sound like just one bad class, one missed opportunity, it sounds like a pattern of procrastination, or backwards priorities. Someone is paying for you to go to school, and you are wasting both that money, but also your time and wasting your potential. This doesn't deserve a spanking, it deserves more then one, until you can start proving you can prioritize the things most important in your life. A bare bottom spanking with the brush or belt before bed every night for a week, followed by 30 minutes in the corner, reflecting on how you will do better the next day, and early bed.

3 exams needs three punishments
The most important thing is to decide what he should do to ensure you revise if you can do them again.
But hand spanking for each exam
Followed by
1. Cane
2, switch
3. Strap.

Then the brush to keep you working hard

Striped
Hands tied above head
Feet with spreader bar
Butt plug inserted
Nipple clamps with weights
Ball gag
50 lashes with spiked whip

Maybe in public place