Short answer? Not much! 

Your account is converted back to the free trial mode: no more than 15 sketches can be submitted to classes. Your students will lose their Plus+ subscription, and will not be able to create private sketches anymore, etc. 

Nothing that's already done will be affected: all sketches and classes, their privacy settings, etc. stay as-is. 

Cancelling your Professor Plus+ subscription only affects newly created sketches: your students will not be able to set sketches to private, they lose their larger file space, etc. 

Never fear! Your class and sketches will not be deleted and students will not lose access once they unsubscribe.