If there are more teachers in the class, you can easily add the others yourself. On the class page, under the Teachers heading, you will find the Invite other teachers button. It opens a pop-up where you enter the teacher/teachers' information:
The teachers you add will be notified by email. If any of them do not already have a Grow Planet account, one will be created for them.
Exception: It is not possible to manually add teachers to classes controlled by an external service, e.g. Skolon. Teachers registered via external services can also not be manually added to classes.