I was just hired to run a day-long percussion clinic at a local HS in which I do not know the excact playing level of the 11 or so kids.  It's kind of a winter concert band clinic.  They want me to fine-tune the music they are working on for the concert band festival they are attending in a few 6 weeks.  I sense that they have the parts in good shape.  I can work the music for a few hours but am contracted to work them for 6 hours.  Any ideas on some things I can address with them? I think that they will be on a pretty high playing and musicianship level.  Thoughts/Ideas?

That sounds like a lot of fun!

If I were in your shoes, here's what I would do:

First off the bat, I would work their concert band music until it is pretty much ";ready to go."; The band director in me would really want me to be sure that at the end of that day, those kids knew that music at as high a level of proficiency as possible. The percussion director in me would want to do a bunch of other stuff...

So, as you go through their concert band music, look for those ";teaching moments"; where, instead of working with only one player on their triangle part, create a mini triangle masterclass. In those times, feel free to teach beyond the printed notes and demonstrate a few different techniques that the students can draw upon to play the part. Even let the other students try that part. I do this frequently when I teach my own guys to be sure that everyone is getting the information -- hopefully, it pays off when someone else gets a part similar to that.

Also, bring a couple of percussion ensembles with you. You will probably want to check with the BD and make sure they are cool with this (";hey, if we're rockin' on our music and need something else to do, do you mind if I do a percussion ensemble with them?";). Then look for those same moments that you can use for mini masterclasses. Also, consider doing a ";rotational rehearsal"; where you let everyone pick a part, then after you've reached the end of a major section, rotate everyone to their right, then go again. It puts students out of their comfort zones without hanging them out to dry.

Then, if you are still looking for stuff to do, have an extra topic you can go over with those students. Something like certain technical aspects of playing snare drum (improving your buzz roll, having a plan when it comes to sticking, etc.), or four mallet playing on marimba (either introducing it or refining it) -- a topic that is likely to help each percussionist no matter what they participate in.

Good luck! With this sort of thing, you just want to over-prepare. You can always have stuff you didn't get to, but never run out of things...