I’m not sure of a per-student budget, but I think you need to look at what you actually need to teach. My budget changes yearly based on what I need, not my number of students. I’d break it down by class or level, and see what you need to get the job done. Early Childhood is probably is lowest-cost. I’d invest in the First Steps in Music for Infants and Toddlers for your EC, then for Preschool and Beyond for your early elementary grades. If you need to buy books for the rest of your elementary classes, I’d add in that, plus classroom instruments or other things you will need to teach them.
It gets a little harder for the older students- do you need to buy method books, or do parents buy them? Do you need school instruments? Percussion instruments? Think of how many pieces each of your groups will perform for each concert, add them up, and add in a little extra for some wiggle room. Do your students supply their own instruments for rock band? Do you need to purchase music?