What corrective exercises would you give to a student with sickled feet?

Tendu. But I would have to be there to make sure the student truly understood what I'm after.

Another good exercise is to simply practice a lot of sur le cou-de-pied from 5th position. It's a great exercise to help shape the foot. I think you also need to watch the student's feet and remind them often to not sickle. They need to eventually be having this awareness on their own, but in the beginning they need extra help until it becomes part of their consciousness.

thank you citibob and gina. you have been most helpful as ALWAYS!
i find also with younger students (have also used with older students) if I stick a sticker on the inside of the ankle and also one on the knee, I can kill two birds with one stone...correct tendu (star on ankle to face the audience, star on knee to face to the side), brilliant also for working on the elusive turnout. (along with concentrating on posture, alignment and technique at the same time of course)