|MadSci Network: Microbiology|
Nowadays, agar is used as the solidifying agent to make bacterial growth medium for petri dishes. Agar is sort of like gelatin but it is made from seaweed instead and is stronger and firmer. It generally has better properties than gelatin for microbial growth.
Agar is not a controlled substance, it should be possible to buy some. You might check with your science teacher at school to see if there is some way they could help you get some.
If you can't get any agar, then it would be possible to use gelatin. A very long time ago gelatin was used as the solidifying agent before agar. It isn't as strong or solid, and some bacteria can actually degrade the gelatin (unlike agar).
You could probably use plain gelatin from a grocery store, dissolve it by boiling or autoclaving, add the other ingredients for your media, and pour into petri dishes.
You need to be sure you are adding the appropriate nutrients to your medium to ensure growth, like yeast extract and peptone, or nutrient broth, or similar highly enriched growth media.All this should be available from any scientific supply house, such as wherever your school gets their supplies for science labs.
In terms of experimental design, I would sample each persons hand first while dirty, then again after washing, and and present your data as a percentage of bacteria still surviving. The control would be washing with water alone. Be sure they hadn't washed their hands before you do your sampling for a few hours.
Try the links in the MadSci Library for more information on Microbiology.