This is built on the following conditions:
-Cannot be in a "firing" state on school grounds
-Must never be used to shoot/harm/threaten living beings
-Must bring in own materials, OR use materials in the sheds (there is plenty..)- i.e. no ordering in new materials
-Springs, nuts, bolts, air fittings, valves, o rings et cetera must be sourced by me.
Obviously these are all quite easy to satisfy, so it will be built!

About the design:
Hammer valve, actuated by a bolt in blowback configuration
Bolt "selects" ammunition and reloads via bolt position
Trigger and sear with side bolt for manual re-cocking
Only uses 2 springs, with an easy to tune design
Modular and serviceable
Can use magazine, hopper et cetera
Can fire bb's or pellets
No need for airtight mag or hopper.
Here's a quick concept drawing:

From left to right:
- Front plate, barrel and hammer seat (hammer cover not pictured)
- Hammer valve
- Bolt
- Side bolt (manual recocking)
- Body
Now due to the limitations of google sketchup (can't push/pull curved surfaces) the air inlets on the bolt and hammer are missing, but they will be there!
Can anyone see problems with the design (see link above for workings). Note that the hammer valve has been centralized (thanks to JSR)
Thanks guys