One step further, let non-administrative users delete their own versions only. That should handle multi-student Wikis, non-administrative teachers, TAs, etc. Plus then you don't need a group check on delete necessarily. If your rights system will allow it why don't you encourage the teachers to restrict a page rather than deleting it so that they are protected from their own mistakes.
How much harm really comes from not supporting delete though? Most Wikis would prefer that a page be orphaned and ignored rather than deleted.