Hello, BadlyDrawnGirl...If you have had injuries in the past, it might not be a bad idea to have a physical therapist take a look at your ankles and give some input and perhaps create some exercises designed specifically for you. From the dance end of it, lots of tendus and eleves!

You might also try using a theraband. Sitting on the floor with your legs stretched before you, place the theraband under the ball of your foot and flex and point your foot. Write the letters of the alphabet up to G, too, to add some circular rotations for your ankle. Here is also something to think about. When you take your class, every time you rise to your 3/4 pointe, make sure that you are
always using your full potential in this position, making certain that you are not sickling your feet. Make certain, too, that you are performing your piroettes on a straight supporting leg, "pulled up" in the supporting hip, and maintaining turnout throughout the entire duration of your turn. Good luck, and hope this helps...