A lot of this will depend on what level your students are at. How are they on mallets? Are they advanced for Middle School students or average?
To this, I'd add: what instruments do you have available?
- Mallet instruments: marimba/vibraphone/bells/xylophone/chimes?
- timpani - two? four?
- hand drums and "toys," like congas, shakers, guiros, triangles, etc.?
This is a tough challenge, finding percussion ensemble music for middle-school-aged students that 1) isn't too difficult, 2) doesn't call for instruments you don't have, and 3) doesn't sound completely patronizing. (Note: that's not a comment on any of Bart's suggestions; I'm not familiar with any of the compositions/arrangements he has mentioned, mainly because I haven't had much reason to check into percussion ensemble music for this age group. It's just that there's a lot of crap out there geared towards this age group...)