If you already have a teacher account linked to a school, go to "internal volunteering", on the upper right corner. You will then be able to add events (short or long term).
If you do not have an account yet, write to firstname.lastname@example.org with your school manager in copy so he is also aware of your subscription. The account will be opened in less than 48h, or even less if you are in a hurry!
CAREFUL: If you register by clicking on "I'm looking for volunteers", your opportunities will be seen by everyone, which is problematic.