Basically a few ways to go about it:
1. Tell her to stop titillating you by dressing so skimpily. You are still underage for sex.
2. Gimme her no. I ask her to read sgForums.

3. Tell her in your most charming tone.. " baby, I tire of your little games. Each time you come, you titillate me to errection. My mind is always on your boobies. Let's just have sex and get it over with!"
LOL you lucky guy. I still remember in Secondary School we have this pretty relief teacher who took us for A and E Maths.
Even though she is quite flat chested, she likes to wear tank top or spag to school. Some of us would pretend to need help during classwork so that she would bend over and teach while we take a good look.
Haha.. But alas, that didn't last cos she dressed more conservatively after 2 weeks.. Probably figured out what's going on with all the laughter whenever someone did that.