Cut class
To not go to class. I cut class last week and went to a movie.
Education Idioms
Catch on
To understand something, to learn about something. I was finally able to catch on and understand the math problem.
Crack open a book
To open a book to study. I did not crack open a book during the weekend.
Crack a book
To open a book to study. I did not crack a book until the last week of classes.