When the teacher marks a student as "seen" in the main teacher view, and notifications are enabled, he receives a notification e-mail that the appointment has been "cancelled or moved". This is apparently incorrect.
The e-mail is generated in the function scheduler_delete_calendar_events() in locallib.php. It seems that this part of the code is executed only in the above case. (The notification sent when a student cancels the appointment is generated in a different part of the code.) In any case, it seems highly unexpected that scheduler_delete_calendar_events(), which is supposed to modify the calendar, has the side effect of sending e-mails.
I therefore propose to remove the part of the code that sends this e-mail; see patch attached.