You can also keep a change of clothes in your gym locker. No one can tell by looking at you that you have your period. You don't look or act any different. People will only know you're having your period if you tell them. You can still swim, play tennis, bathe, and do all the things you usually do.
Periods stop while women are pregnant.
In time, your period will stop for good. Usually, it stops when a woman is between 45 and 55 years old. Most girls don't have serious problems. But be sure to tell somebody if you have really bad cramps, if the flow seems very heavy, or if your periods don't become regular. Tell your mother, the school nurse, a teacher you trust, or your family health care provider.