I would be flattered, and think it could be a really cool opportunity (not that this would happen)!
I completely agree with @Ryuson about the value in talking about healthy relationships as well as just sex education.
My message in brief would be that sex can be an awesome thing but that it needs to be done with communication, consent, and consideration (for yourself and your partner(s)). I'd talk about the many variations of gender, sex, and sexual orientation and generally cover some of the wide ways in which people find pleasure with themselves and eachother. I'd talk about masturbation (male and female. I would spend a great deal of time talking about consent and communication with your partner about sex (I don't think this was really covered at all in high school for me). And then I'd focus on sex safety some as well, not just contraceptive options, but the risk involved in various sex acts and how to mitigate that risk.
Most of all I would make sure there was plenty of room both publicly/anonymously in class, and privately afterwards for anyone to ask questions.
I'd make sure to have plenty of visuals and handouts, lots of "to find more info go here" kind of links, as well as key numbers to hotlines, clinics, and other things that might be handy for high school teens to have whether or not they are sexually active.
I feel like I had a pretty decent sex ed course in high school, we didn't have a half-baked abstinence only program, but still I can't credit it with being super helpful beyond the basics.