1) pair up kids
2) Coordinate with teacher so every thing is ready before hand - you don't want to spend time in setup. (arrive early, scope out the space)
3) there is always 1 kid who is "smarter" than the others who is going to cause a problem - don't get distracted by them (or have a plan)
4) Have everything that you want to say in a powerpoint or something that can be displayed using equipment at hand - avoid wasting time writing.
5) Keep presentations short ( ~4min max) before activity.
6) More kids: more chaos; longer to get going. Cut in a third the time available - that is the actual time you will have for your project.
* They don't pay teachers enough. I can't stand getting my own kids to get work done. Let alone 30 other brats.