Most ballet exercises cover a lot of muscle groups, while exercises to prepare for pointe work, or foot work in general focus on the feet/ankles. Here is a simple exercise you can use as a warm up before class that will activate your posture, leg muscles, and foot muscles. It will get your joints more flexible in a gentle way as you increase your circulation and alert your brain/body that class is about to start!
As an adult ballet student you may have been sitting at a desk all day, or confined more or less, in an office venue. Coming into an adult ballet class, usually at the end of the day, your body is ready to wind down, yet - here you are.
Prances is a wonderful warm up movement. Facing a mirror, you can set up your posture, checking that your legs are truly parallel, with knees over the toes, not turning in toward each other at all. You can also check that your hips and shoulders are level. If not, be aware of tense areas in the body as you warm up. (And check at the end of class to see if you look more level).
So if you haven't learned prances, start by pressing up onto demi/three quarter pointe and then come down on one leg through a flat point, and then lower into demi plie. The other leg bends, and you can press over a little on that (now) working leg, to get a little more flexible in the ankle and toe joints.
Slowly transfer the weight onto both feet as you press up onto demi pointe. Check that your neck and shoulders are not working, and that your core muscles, or lowest abs, are controlling the neutral pelvic posture (pelvis tilted neither forward a little, or back).
Do 12-16 slow controlled prances, then take a little break. Now turn sideways to the mirror, repeat all, checking that your demi plie is going up and down a plumb line, not allowing your weight to press back at all at the depth of the plie.
This is a great yet simple warm up. You can add to it by doing a port de bras - bring the arms up from low fifth through fifth in front to high fifth and down through second position. This will warm up the back and shoulders. Breathe deeply and easily.
This little preparation to class addresses the basic foot movements for ballet - and every use of the foot is an exercise to prepare for pointe work, every single one. Flexing and extending the ankle and arch properly, while controlling your posture, and getting a little flexibility through the toe joints (especially the big toe), is no small feat.
When you do eventually get into pointe shoes, you can do the same exercise as a preparation/warm up, holding onto the barre, and going up onto pointe and coming down through the foot, changing the weight, and pressing over your pointe a little if you need more flexibility. If you have hyper-mobile ankle joints, you can practice holding it back a little, staying on your pointe shoe platform instead of pressing over.
I hope this will help you enjoy your adult ballet class even more!
If you want to go the extra mile, get this book to prepare for pointe shoes.