This is usually caused by contractions in the uterus that can often feel worse if you call in sick. Remember that your period is not an illness; it is a completely natural thing for your body to do. Keeping active will help these cramps to ease. So do take part in school gym lessons and don’t slack off on your normal exercise regime. Just take it a bit easier than normal and do try and include a few more relaxation exercises. If you have to take painkilling tablets, stick to the instructions on the packet and don’t take more than the prescribed limit. If none of this helps, you should talk to the school nurse or doctor.