alt Hacker NewsMIT does this. Every class starts 5 mins past, and also ends 5 mins early.
We did this at Google too while I was there (only the started 5-mins past part). It works really well.
No need to change the Calendar events though. It's just implicit that we'll start 5-mins past. (Or, well, explicit in MIT's case).
When I attended Lund University in Sweden, classes always started at 15 minutes past the hour, called "akademisk kvart" https://en.wikipedia.org/wiki/Academic_quarter_(class_timing...