The other answers are all great! Here are some additional items that might be found:
Theater/Theater class: Various costumes, usable as disguises or able to be salvaged for the cloth they contain. If the tribe has an encounter with superstitious people, a well-planned scare with grotesque costumes from the Halloween closet could be just the thing to keep them away. Also, costumes could serve as status symbols or just help with the general political theater - Freemasons are famous for doing this with the funny hats and robes and medals and whatnot. Why does the Chief wear wizard's robes? Well he just does, that's how we know he is the chief and whatnot.
Music class: Plenty of instruments, probably out of tune but quite possibly fixable. Use them for social purposes within the clan, or send clan members out to other towns as traveling minstrels to earn money and gather knowledge. Unsalvageable instruments can be broken down for materials (brass and wood, mostly).
What, no one in the clan knows how to play the tuba? Sounds like that's a quest a-brewing. Send a bright young aspiring musician out on a dangerous journey to seek out the legendary Brass Band Master who is rumored to be living somewhere beyond the Green Mountain Pass in a cave next to the Lake of Mineral Valley. Bonus points if his name is not Obi-Wan Kenobi.
Lockers: In addition to ransacking the contents as described by others, disassemble the lockers themselves to obtain sheet metal to use for armor plating or construction material.
Vehicles: There are probably vehicles on the campus. School buses are useful for transporting large quantities of clan members at the same time and can be lived in as well. There may also be private vehicles on campus, and perhaps even delivery trucks.
Electrical: Miles of wire, transformers, fuses, gauges, lighting. Remove the floodlights from the football field for an astonishingly powerful blinding weapon.
Records: High schools tend to produce a large quantity of written documents pertaining to student performance and behavior. Schools are increasingly becoming digital, but a rural school could be behind the times and, in any event, paper records are almost impossible to eliminate completely. These could be helpful for establishing, enforcing, or challenging a social order - "See, my grandfather did get A's in Political Science, History, and Geography! I am the best person to be Chief!".