You can only register if your school is officially taking part in the event. If so, you should get an access code card from your teacher. You need to enter the access code from that card, and pick your own username and password – write them down so you don’t forget them because you can’t log in without them.
If your school is not taking part, go and nag your teacher to sign up for the next event! If you are a teacher you may want to nag your Head of Department. You can register your interest in the next event on the teacher sign up page.
But don’t give your teacher too much of a hard time – spaces are limited, maybe they did try to get you in. Get them to apply for the next one.