Gas. Somehow we leave the gas taps on and light it on fire. If it doesn't ignite and explode then it'll at least suffocate some people.
The issue with this plan is that it will be centred around the science labs and that is not the target area.
C4 or other detonated explosives would work because it would allow me to target a specific area for example Mr Halls office. It would also allow for damage control to prevent destroying the entire building. Issue: its illegal and hard to come buy and expensive.
Mix bleach and ammonium hydroxide. Will create chlorine gas and kill everyone. Pros, easy, cons, not explosive!!!
Make nitroglycerin.
Make napalm.