Yes, you can join a school at any time.
1. After logging into your account, select the profile icon in the top right corner, then 'Join school'
2. Enter the school key, then choose ‘Activate’
Please note:
Classes created under ‘My private teaching’ will not be transferred to the school account. To get more information, please read I’ve created classes as an independent teacher and then joined a school. Will my classes be linked to the school?
If you do not have a school key, please read How can I get a school key?