My university had a simple fix for this problem - all lecture halls actually had doors at the back. People who have back-to-back classes in different department

My university had a simple fix for this problem - all lecture halls actually had doors at the back. People who have back-to-back classes in different departments had to take a shuttle bus to commute since the campus is big. They could slip in silently from the back door and take a seat somewhere in the last few rows. No body gets disturbed, and the prof doesn't get offended.