Just say "protect yourself."
I thought I hallucinated this back in Junior High (what we call Middle School now) but no, The Walt Disney Company made an anti-STI educational flick back in 1973 that was quite frank even for those liberated times. Here it is, thanks to Google Video:
Here's some background on it, from Other Cinema.
Of course, such frank, educational, science-based information, told in an entertaining manner, is not par for the course nowadays. There has been a return to the frightening, something that was already happening back when I had Health Class in 1976, but which started full-force in the "Just Say No" Reagan era. The only thing that compares is the current campaign of the LA Health Department against the rebound of Syphillis in the Gay Male community.
And even if you did teach teens in a frank, science-based manner about STIs, you'd lose your federal funding for health education in a twinkling. Why? Because the Feds now will only fund ABSTINENCE EDUCATION. Thanks a lot, Reagan and the Bush Family. Something that's been proven not to work, and to actually make matters worse when teens and young adults succumb to their hormones. Something that's been keeping us in the Third World column with regard to teen births and spread of STIs.
If you are a parent or even an aunt, uncle or cousin to a vulnerable teen or young adult, you should be really concerned. And get really angry. And speak up.