Heh, guess I'm kind of spoiled about the sex thing

. But I know exactly what your talking about with the whole health teacher thing. We took 2 days to go over why you shouldn't have sex and stuff and those teachers are very tactful in how they deliver their message. I'm sure STD's were brought up and extremly over exagerrated.
Example
Me: Well if you use a condom what is their to worry about?
Teach: Well say she has a syphilus sore that goes all the way up to her stomach, you'll still come in contact with it even though you are wearing a condom.
(Ok honestly if a girl has some huge ****ing sore going all the way up to her stomach from her pubic area I am not gonna mess around with it in the first place

)
They are big on making girls out to be sluts to if they do have sex.
My city has like the third highest teen pregnancy rate in the nation, or something like that been a long time since I've heard about it and can't remember exactly. So that does make the whole "consensual" battle not as bad as some have it.
I would approach it from the angle of not anticipating sex on any given evening. If she's your girlfriend of 4.5 months I'm sure you two are showing strong physical attraction to each other. You just need to get her alone, something kind of romantic (not in a car, and not in a party with 15 people standing outside the room.) Parents going out with friends for dinner and a movie is a good oppurtunity to take her back to the casa.
Set the mood, take her to your room, put on some good music (John Mayer

he never fails for this kind of situation.) Start making out and etc and slowly progress things. Reading the bibles articles on fore play is a good place to start. Turn up HER horniness.
If you really want me to elaborate further on once you get her in this situation I will but I have faith you can figure it out plus that post would be just as long as this one and I'm to sick and tired to keep on typing.
Good luck to you bro.