This means you have not updated the Tutors area ready for the new year.
To update this please go to Admin > Config > Tutors
For any Teachers not currently assigned to a Form, click Assign.
Then, simply chose Year Group and a Form for that Teacher and click Update.
If your Teacher is already assigned to a Form then click Manage.
Then simply click Unassign to remove the current allocation. For the new allocation, choose the Year and Form from the dropdown and click Add New.