When I was in middle school we had a smallish
theatre that was in the oldest part of the building, and although it was a nice space it didn't have catwalks or a "
grid" accessible by any permanent means. We had a 16' double-sided fiberglass A ladder to access the electrics above the
stage, and an 8' aluminum (argh) A ladder to access the beam lights above the lip of the balcony. As a middle school student I was allowed to set and use these ladders. I was among the few students though whom the director trusted to use such equipment - others would have gotten trouble.
When I moved up to the high school, we were in a BEAUTIFUL new building (built in '01) with a huge professional
theatre. We had three catwalks over the
house which were very safe and enclosed if you stayed in the typical areas, and had ample head-room with no low clearance areas. Of course us techs went above the catwalks for some beam hopping and behind the stairs in the box-booms where there were no railings to store very infrequently needed gear. We also had a load
rail catwalk at 58' above the
deck that we were of course allowed onto. That was among the more dangerous of the catwalks since it had a side that was more open than the rest to facilitate weighting. Us student techs were allowed everywhere but of course actors and general drama people were not and were quickly yelled at if they attempted it for kicks.
The building that USED to be the high school and is now the middle school (the building I attended is now gone, RIP) had a very fun, intricate
system of catwalks above the
house. It was accessible from both
stage right in the scene shop via a series of shipman's ladders totaling about 30' in height, and from the booth in the upper-house section. Being an older building this
system had its quirks - low clearance in lots of areas, trip hazard where support beams cross the grating, crap lying around that could be kicked down into the
house by accident (did this once, nobody got hurt but it will hopefully be the last time I create such a hazard) and spots where you had to walk on thin but reinforced ceiling concrete and
HVAC ducts to get at things. Students have historically been allowed up there and to my knowledge still are. Once again only trusted students but beyond that, no specific rules, and no rule written anywhere in my former school district.