Have you been having unprotected sex? If yes, plz take a home pregnancy test and confide in your parents/ trusted relatives or a trusted school teacher etc. Also, educate yourself about the menstrual cycle, and about methods of contraception. Visit Planned Parenthood or anything similar in your country.
Sometimes we skip periods even without pregnancy being involved due to stress, or if you've started menstruating recently. A lot of teens miss their periods in their early teen years.
Wish you all the best.