I have a real problem with the mentality that some kids have in thinking that once they've "made it" on an instrument, they're assured a spot on it in the future.
I actually had a snare drummer last year who busted his butt all winter to make snare and as soon as he made the line, he totally slacked off. Also had a similar problem with a tenor player (though he actually put some effort into it once it was clear that he was holding us back).
Anyway, I've laid down the law already for next season: consider your position to be probationary because the moment you start slacking is the moment you'll find yourself on the sidelines until you get your act together. Disrespect for the group (which is what slacking-off amounts to) will not be tolerated.
And hey, while I'm on a rant, what is up with people being so down on playing in the pit?!?! This mentality definitely exists in my battery players (even in the pit players themselves) and it drives me nuts! If you're into the music, playing in the pit can be just a rewarding as marching in the battery. If you're a good keyboard player, it can be a blast. I'm determined to turn this self-defeating attitude around. The battery players have got to understand that their success as a drumline is ultimately going to come down to whether or not they have quality playing in the pit (this, of course, assumes that the drum judge is at all competent). The more actual percussionists that are in the pit, the better it is likely to be (no offense to all the excellent mallet players who play a woodwind in the off-season).
To CHSGIRL's issue, I do think the instructor could've maybe handled the situation with a bit more consideration, but I have to say that I'm in agreement with Project_BP. It wouldn't be fair to the other 5 bass players who have proven to work well together, to split them up and knock one off to make room for a previous year's veteran (unless doing so would raise the quality of the line).