Needing a restaurant between Houston and San Antonio on I10!
I'd also check out the Schobel's in Columbus. The buffet there is supposed to be quite good, and they're accustomed to large groups. Ordering off of the menu isn't getting much love these days, but the buffet is still getting great reviews, and a bunch of band kids could zip through that buffet quite quickly, with everyone getting what he/she likes best without having to worry, and wait for, a server. I'm sure they'd negotiate a better price for a group that large, and the bus drivers and tour escorts will undoubtedly eat free.
I'd definitely suggest you give them a call and see what you can work out.