Event absences triggers an email?
I don't think there is a straight forward way to do this.

It may be possible if you register the entire class list for all the classes (events) and then set the participant status to "No-show" when a student misses a class.  Then you might be able to do something by shuffling students between a series of groups - "no-show this week", "no-show this week &amp; last", "no-show three consecutive weeks" using CiviRules and sending a weekly "Constituent summary" report to the teacher restricted to the people in the "no-show three consecutive week" group.  

However, if you have more than one class you would need three groups for each class so you could get to an unmanageable number of groups fairly quickly.

Also, I am not sure if the existing conditions and actions in CiviRules would be enough for this purpose or if you would need to create additional conditions and/or actions