It's also a good idea to see a health care clinician. You can carry a supply of pads or tampons in your bag when you think your period is coming. Ask your school nurse or teacher for them if you forget. Don't be shy. Remember, all women have had periods.
Some schools have machines that sell tampons or pads in the girls' bathroom.
Public bathrooms often have them, too. If your clothes get stained, you can wrap a sweater around your waist or ask to go home. 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.