The 10 minute rule, and in some cases the 5 or even 15 minute rule, is an unwritten rule in schools, colleges and universities.

The rule is used when you have a class/lecture, and the idea is that if the teacher/lecturer is not there in the classroom within the said time period (5, 10 or 15 minutes - depending on what has been decided) then you can assume that they aren't coming and you can therefore leave to do other stuff/go home. This could also possibly be used in the workplace for meetings. (change 'teacher' to 'boss')

N.B. This rule can also be used/changed to deliberately skip classes. E.g. if you know that a teacher will be 10 minutes last, you can switch to using the 5 minute rule and leave before they get there, thereby skipping the class.

Got the covid-19 vaccine