I'm not the coordinating teacher at my school. How can I add licences for the class activities or modify the registration to the contest?

You will have to be assigned the "admin" role by the coordinating teacher of your school. 

The coordinating teacher of your school will have to enter his or her account and:

1- Search your name in his "Colleagues" list

2- Click on your name

3- Click on "Change role" on the right hand-side corner

4- Give you the "admin"  

Like this you will have the same rights as the coordinating teacher of your school.