In addition to what's covered above, it's a good idea to have feminine products ready for that first period, and to know how to use them. If you have a pad with you, go to the girls' room and put it on. If you don't have one, go to the school nurse, the office, or wherever you can get a pad. If you don't need it, maybe one of your friends will and she can return the favor some day soon. It's probably just a heavy flow, which can happen during a period's first day or two.
Some girls normally experience a heavy flow.
If you have a prolonged heavy flow, call your doctor. These days it's hard to use a period as an excuse to get out of gym class. Believe it or not, the more active you are, the less likely you are to have cramps. And, if you're worried about leaking during sports activities, check out the Kotex products section of the website to find out how to help prevent leaks by using a pad or tampon that fits you and your period. Cycles range anywhere between 21 to 45 days.