This means you have not updated the Tutors area ready for the new year.
To update this please pop 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 it from the dropdown and click "Add New"