Looks fairly good as a start.

Suggestions for your pack:
-Dust mask- N95 is very good, bandana works pretty well to
-Work gloves
-Spare glasses if you need them- if you don't need glasses, a pair of safety glasses anyway. And if you wear contacts, regular glasses are a must, contacts + dust = pain.
-Spare socks
-A couple of contractor bags or large garbage can liners- get chummy with the janitor (good person to know in the first place- they know the building better than anyone)
-Duct tape, rewrapped
-nitrile gloves- blue medical gloves
-couple zip ties

I think you might need a bigger backpack. :P Actually, most of it is pretty small. The contractor bags fold up to the size of a piece of bread, nitrile gloves to almost nothing. Most of it will fit in a plastic pencil box. Your socks and work gloves will be floating about, but that isn't a big deal, they are pretty flat to pack.

For your pocket:
-flashlight
-sturdy toenail clippers- useful if you can't have scissors or wire cutters

Unfortunately, given that you are 13 and this is for a school, a lot of the other items get dicey. Knives are out, as you said; so are most tools. Either mistaken for burglary tools or as drug paraphernalia (I can't see someone using vice-grips as a roach clip, but someone in administration might be dense enough to do so). Even an empty water bag might get mistaken as something for someone using inhalants. *scowls, muttering about over protective paranoia*

All that being said, without giving too many details, what is your school's plan for this scenario? They'll have one, and they have to be able to provide you with a copy. The best thing you can probably do is sit tight, and keep cool. You sound like a scout- remember the leadership methods you've seen. Peer pressure can be a good thing, particularly if your "adult supervision" is flaking out.