"What would be the leeway cut off point then? 8.51? 8.52? What about the person who turned up at 8.53 and said, "but it's only a minute after the minute or two leeway!""
Now you will see the wonder of mathematics. 
Let us propose a leeway system where the allowed leeway time halves after each missed leeway point.
So, after 8:50, leeway until 8:51 say. 1 minute leeway.
After 8:51, 30 seconds leeway, takes us to 8:51:30
After 8:51:30, 15 seconds leeway, takes us to 8:51:45
and so on (I'm starting with one minute because sums are easier!).
So, the question is folks, with such an infinite number of leeways, what is the time after which the leeways no longer apply and you are finally, officially late?
Helps with parents, you can truthfully say that the school has an infinite number of leeway points...........That you have just missed all of them...............so are late as well as feeling pretty shit for having missed an infinite number of things!
Or perhaps why they don't let the maths teacher rationalize the timetable and make everything more efficient...........................