I have changed my classes for the new year, but my reports show the old teachers

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.