I had always had a crush on my Physics Teacher. She was in her mid 30's, pert, always wearing long skin-tight skirts and tight blouses.
One day I was working on a gravitational experiment - our Physics class only had a few people in it. I wanted to get the results done, and asked the teacher if I could stay back for 20 minutes. She said that was no problem, and she'd do some marking until I'd finished.
That day, she was hotter than usual, and had her 2 top-most blouse buttons undone. To a horny 17-year-old that was a major turn-on. The desks in the classroom were very large and since you sat under them, from the teacher's perspective all she could see was the top half of you. With blinds on the windows, the room was pretty 'private'. As I began taking samples from the experiment I glanced at the teacher and saw straight down her open blouse. She had a fabulous tan and since she was bending downwards over the desk, her boobs looked like 36 D's.
In what seemed like a nanosecond, my cock went from flaccid to rock-solid at this amazing sight. I had a few tissues in my pocket and couldn't resist the urges I was having. I clumsily used my left hand and slid the tissues around the top of my penis and turned slightly away so if the teacher looked at me she wouldn't notice my left arm moving about.
My eyes kept focused on her chest as I slowly jerked under my trousers. Several times she looked up at me and smiled, which made me tug furiously at my throbbing member. I knew I was about to come any second and when I did I couldn't help but let out a kind of 'high-pitched' 'arghhh'.
As I did the teacher asked 'What happened'. I still had my cumming cock in my left-hand and unbeknown to her what I'd done, replied 'I had too much load' (measuring gravitation on a dolly going down a ramp you see!)
A few minutes later I told her I was finished and packed up the experiment, managed to get the gooey tissue out of my farah trousers and had a smile to myself for the last few weeks of that term. That was 10 years ago - I saw the teacher last year and now in her 40's she's just as sexy and I still jerk off on occasion just thinking about that first and only school-based wank.