If you first chose to teach independently, you can join at any time. If you'd like guidance, read: Can I join a school if I have already chosen to teach independently?
When you first log in after completing the registration process, follow these steps:
1. Click on 'Complete account set-up'
2. Select ‘I am a teacher in a school’
3. Then 'Join a school'
4. Enter the school key provided by your school administrator and then 'Activate'