For middle school to younger high school kids, a 5B or 5C is a great choice. I would recommend having a 3C or two around for your older kids as they may find it more comfortable. I tend to prefer a 5C to a 5B unless it fits the kid better. While there is no one-size-fits-all mouthpiece solution, this is a good starting point.

But the OP is a trumpet player/music major. Their needs are much different than those of a band director putting together a set of Or recommending school equipment. I wouldn’t recommend a MM for anyone to buy for school equipment. It works for me personally. The OP should be making this choice with their trumpet professor, who has a much better handle on the OP’s playing, needs, and challenges.