One way to practice viola would be to find an alternate ensemble. I'm not sure about any orchestras in the area aimed at middleschoolers. It occurred to me that with two friends doing stringed instruments, you almost have enough for a string quartet. That's 2 violins, a viola, and a cello. Maybe you can entice the music director to work with such a group after school, if the kids can put one together. Btw, the baritone might be a referring to a brass instrument that looks like a tuba but is half the size. It's sometimes interchangeable with the term "euphonium." It's a very pretty and difficult instrument.