You can do a lot with copydex (ie latex) mixed with cotton wool and acrylic paint (just make sure kid shaves that spot first to avoid the 'waxing effect' when is pulled off... ouch)
It makes a form of budget foam latex. Good for crating goblin effects.
Blood mixtures are often golden syrip with red, a little yellow and green food dye and bi of flower
Now of you sploge this onto tissue you can make a kid look skinned (add black here)
And other creativity... ie sofened noodles or spagetti for veins
Condoms with water food dye and newspaper for intestines
(this also helps remove the too shy and embarrased to go by condoms from the pretty chemist assistant when old enough to have the proper use for them when having the excuse they are for an art project... syndrome
Or how about punk hair styles...
Joicos Ice Spiker is excellent for this on hair up to 4 inches or I think Bedhead do one
Stargazer do great uv glow hairspray whihc works on dark hair
Alginate can be used to cast body parts which can then be used for say costume parts ie cast your hand then use this to make a monster glove.
For older students say aging wood.
Tea staining paper t make scrolls or pirae treasure maps
Maybe wih secret messages (think you can use lemon uice for invisible ink or similar)
Bit more fun then what I had in school and actually some of that is actually used in the film industry.