My daughters MS director wants all of the students to switch to a Bach 3. Is that right?
Mouthpieces come in sizes like shoes do. Lips come in different sizes as well. Besides lip size the strength of the embouchure also comes into play.
Some highly respected playing professionals play mouthpieces the size of a 7c, some smaller and of course some larger.
Your daughters director feels that he should say something about mouthpieces but he does NOT know what to say. Most Band directors do not play every instrument well. Some colleges cover all instruments in 1 semester brass, woodwinds and strings. Some do not teach them at all.
There are some playing tests that can help determine if your daughter needs to change her mouthpiece. One is how well she plays below low c. If a mouthpiece is too small the low notes do not respond very well. However as a second year student she may not have spent enough time on low notes to play them well on any mouthpiece.
I would think that a copy of Clarkes technical studies $ 10 – $ 12 would be a better investment at this time. 2 or 3 weeks of playing this will help you to determine if she needs a bigger mouthpiece.
As for cost Bach, and Schilke mouthpieces cost about $ 30 in mailorder from several places.