I teach a girl who bites her cheeks when she plays flute.
We've found it's when she plays staccato and mostly higher staccato notes. At one point i thought i'd solved it by telling her to tongue closer to the front of her mouth but its come back. She can't play for very long without having to stop cause it hurts so much.
I really don't know what to try next!

Have you checked wherther she's tightening her lips to get the high notes out? I think you probably just need to encourage her to use a looser embouchure generally, and not to tighten it to get the high notes/staccato. If she uses a more continuous flow of air it may help as well.

We've done loads on relaxing the embouchure for higher notes etc. and i think she just forgets to do it.
As for the continuous flow of air, that is a big prblem =/ We've done tons on breathing but her breathing never improves =/ She tends to breathe every bar, no matter how many times i stop her and say to try hold it longer. Once i spent about 15 mins telling her the correct way of breathing, i.e. your shoulders shouldnt move your stomach should, showed her, then got her to breathe in slowly using her diaphragm and to see how long she could hold a note and she noticed a difference, to make it fun we even did 'who can hold the note the longest' competitions (obviously not for very long or we'd collapse!!) and as i said she noticed a difference, so i then said 'great, ok so imagine youre about to start a long note as the first note of your piece, take your breath' *she takes a big quick gulp of air and her shoulders come straight up* i was very disappointed !

Other than the cheek biting and the really big breathing problem she's doing well, but i've tried explaining in every way i can think of =/

Sorry for the long post! I'm quite new to teaching so would be very grateful of any suggestions

Have you tried getting her to think of it as sighing? She might be thinking of it more in terms of trying to force the air out rather than just releasing it. You could ask her to try keeping some air in her mouth as she's playing, not chipmunk-like, obviously, but enough to keep her cheeks away from her teeth.

Do you know how she practices at home? If she's sitting at home/slouching/crossing her legs or whatever it might be causing her problems, and preventing her from getting used to supporting the sound. You might want to avoid talking about it in terms of holding it for longer, from the sound of it she needs to take in more air and support it as it's released rather than anything else. If her shoulders are visibly lifting it's a definite problem, what she probably thinks she's doing is completely filling up with air, when she's actually just taking air into the top of the chest cavity, stopping it and getting far less air than she could. Make sure she's aware of it when her shoulders rise, maybe get her to look in a mirror as she breaths in. You could try putting one end of your flute case (after taking your flute out of it) against the wall so that it's about stomach level then lean against it and breath in. If she's taking enough air in and supporting properly then it should be fairly obvious (I know, it sounds completely barmy but it can help). You could get her to try huffing notes as well i.e. not tounging them but using the diaphragm on each note, chances are she'll fall back to tonguing them or may try using the throat to force the air out so it's better to do it slowly and make sure each one is properly supported has good tone etc. I think playing while standing on one leg might help as well, but I can't quite remember why. You might want to make she's keeping her neck/throat relaxed as, if she's trying to use her throat to control/put out the air then it's likely to work against the diaphragm, if that makes any sense - they kind of cancel each other out, it'll also cause tone problems later on (gives a slightly odd/hollow sound).

I think the two problems are probably linked so sorting one will at least help with the other. I still have problems doing the whole support thing, it's just something you need to keep reminding her about.

My teacher tells me to practise breathing exercises at the beginning and at the end of every time I practise, I breathe in and then see how long I can hold the note for. I do this 3 times before and 3 times after. During the early stages she made sure I did this right by making me do it a couple of times at the beginning of each lesson.
My record is 27 seconds!