QUOTE(Lucid @ Aug 16 2009, 12:55 PM)

She should stick with using more mouthpiece as this is the correct way. The higher notes can be difficult to get consistently when you are first trying them so it's not a major problem and she's certainly got time to work on them. You could encourage her to do some extra tonework around those notes. For example she could play an ascending C major scale slurred starting on clarion C (3rd space of stave). The idea would be to gradually work up to the top C without any of the notes splitting. She shouldn't squeeze her embouchure to get any of the higher notes out. Once the C at the top of the scale is working well she can then progress to the altissimo C# and D. Once these notes are working well slurred she can then try tonguing up to them, which is usually more difficult.
I have accompanied Peacherine Rag before and think that the highest note is altissimo D. A trick to help her get the tone of that D would be to play a clarion F (top line of the stave) and while holding the note she should gradually slide the first finger of her left hand off of its key. This should produce an alitssimo D. Once she can achieve the D everytime this way without any splits or squeaks she can then try adding the right hand finger to the E flat key as she slides the first finger off, as that is the correct fingering for top D. Hopefully this will help her get more used to the tone and producing it. Its really a case of practising and doing a little more focused work until the notes can be played with confidence.
Lucid

Exactly the sort of things I was going to say. And I completely agree about the preparation work for the top D in Peacherine Rag.... you don't want it coming out as a squeak instead of a well prepared note! Another thing I'd suggest is really make sure there's a lot of air support behind the notes. It's amazing how much you can get away with "lazy" unsupported air on notes right up to top C but as soon as you go higher to the C# and D a lot more focus is needed.