I'm very much of the belief that with a high school kid, the LAST thing you want to start jerking around with is the mouthpiece when a much better alternative is to examine what's going on with the current gear.
I'm also not a proponent of the oft held (and just as often incorrect) assertion that switching to a bigger mouthpiece is the key to getting more resonance. Better focus and better usage of air gives you more resonance. A Monette B6 compares to a 3C - unless our young poster here has plump lips, this mouthpiece should be plenty big enough to get a resonant sound without having to go bigger. I've seen plenty of kids' chops hosed up from trying to move to a bigger mouthpiece than they were ready for.