All of the above replies are good and
cover most bases.
You need to recruit users from different sections of the school so when you go to ask for maintenance funding there are wider areas to get it from. i.e. dance, darama, award ceremonies, conventions, club meetings.
It is harder to get capital or upgrade funding so put your plan together that only asks for capital funding every other year.
As has been said, look for outside
one off funding and even though it is the easiest solution, keep your hands out of your wallet, that way the
system will find the funding when the increased users group wants more lights working.
A cheap and effective set of worklights, easily switched on, will take the pressure off using
stage lighting for set construction and early rehearsal sessions, with responsability put on a person for every session to ensure lights are turned off at the end of each session. Make sure there are adaquate security lighting on movement detectors or timers to allow people to leave the
venue and feel safe without leaving your main rig on.
I have built up a few
amateur theatre groups lighting rigs and initially we borrowed and loaned from other clubs, juggling productions schedules so shows did not clash and asked the committees to pay 1/4 of what we saved in hire costs into buying new equipment. This built the pool up untill the clubs went solo as they had adaquate rigs to use. You will also find that this method relies on a good relationship with the other clubs and as people move on, these relationships will dwindle over time.