You will only have the option to delete a form if there are no students presently registered to it. To move students out of a form you can either change their form individually on each Student Profile, or by using the Group Updater tool (Admin > Students > Group Updater).
Empty forms will appear towards the bottom of the list, with a greyed out background and a Delete button:
Click Delete to remove the unwanted form. Please note that deleting a form will not affect student data as this is saved against the student.