Teaching Sex-Ed (to any age) can be intimidating and sometimes surprisingly enlightening....here are a few tips from my own experiences...
Never use a banana in a condom demonstration. (The girls will never again eat one in the high school cafeteria..or anywhere on campus for that matter)
You will be informed that the concept of a "one-size fits all" condom does not apply to them. (I spoil this concept when I roll one over my hand and up my lower arm.)
Expect the guys to all talk like studs...and the girls to sound like virgins. (But be wise to the fact this is most likely NOT true for either sex)