Swimming is great when you have your period. It can reduce your crampy period pain and improve your mood. Of course, if you are swimming during your period, you will need to wear a tampon, so you can be prepared for swim school by having some tampons in your PE bag. A pad will not block the blood flow and will probably start to fall apart. Make sure you change your tampon after you get out of the water as it might be a little wet.
Remember, it can take some time for a girl to develop a regular menstrual cycle and some girls can vary by a few days every cycle, even when they’ve had their periods for years. If after 12 months, your cycles are not fairly regular, or occur infrequently or too often (for example less than 3 weeks or every few months), see your GP for advice.