Ride without stirrups a portion of your ride and/or ride in 2 point.
By consistently keeping your heels down
try shortning your stirrups!
Do exercises at home with them. Whenever you're sitting down in a chair at school or on a couch at home, push you're heals down for as long as you can and then relax them for 30 sec to a minute then try again. This will allow your calf muscles to get used to the tension so when you put your heels down in the saddle, it's effortless.
keep them down during your lesson