It is most likely jock itch, which is a fungal rash that develops on your crotch area when that area stays moist for a long time from sweat or water activity. If you are an athlete at school it is very common. Google pictures of jock itch and see if it looks like what you have. Regardless of what it is you need to go to a doc, but if you think it is jock itch your parents will know it is no big deal. They give you antifungal pills and it goes away in a week or two.