Good or mediocre depends on how good
they get.
I would say so long as they had stuff to work on and worked consistently, and learned how to work when they were on their own away from the rest of the line, you would have taught them great. It sucks to be mallet player sometimes, you're always off working on something on your own, and it's really obvious when the player can't play his instrument, there's usually only one (as in, one vibraphone, or one xylophone, as opposed to 30 clarinets or 15 flutes...) and a bad note really sticks out!
I would think your job is really to teach them how to teach themselves.