Ryan,
It has been a long time, but I was where you are when I was taking PE in high school. To make matters worse, I hadn’t even started puberty, so I was short, turtling and hairless. Oh, did I mention fat? Christ! While hairless is all the rage among lots of people these days, it was not cool when I was young. I would give my dick a tug or two as I was undressing (not that helped for more than 5 seconds), then head for the showers. I kept my head down and never made eye contact with anyone. Funny thing is, no one ever commented about it, not even once! I attended the same high school for the next three years after that (took PE as a freshman), and never was it mentioned. I fretted about it every single day of my life, and no one else ever gave a damn. I don’t know if this will help, but I don’t think there is any magic, quick-fix for your problem. Search for the “Never Let It Turtle” thread when you get a chance. Following that advice has led me to the point now where I hardly ever turtle. But, it does take time. Good luck!