Push down in your right stirrup and sometimes do no stirrups to get a good core stren
Use a bareback pad because it slides to one side of you put to much weight on that side. I had problems leaning to the right and the bareback pad really helped with it.
Ride stirrupless!! Oml this will seriously change your riding even after one ride!
Make sure if you're at school you don't wear your bag on one shoulder, this can lead to back problems. Make sure you aren't dropping your shoulder as this is often a tell tale sign you aren't sitting centred. No stirrup work should help too so you get into centre of balance of the saddle
Try and feel for equal pressure on both seat bones throughout your whole ride
ride without your left stirrup and it will force more weight into your right
See if it chiropractic. You may be out in a hip and not know it. I was the same way. Dont get me wrong i still lean, but its out of habit.
I have a friend who has all her weight on her right leg because it's stronger than the other. Try doing some excesses with your right leg to even out the strength
Have you checked to make sure your stirrups were the same length? That would be an easy fix if they happened to be a little off.