For one, traffic lights work. When people obey them, they (excepting a faulty light) don't get into wrecks. They may not be efficient, but they get the job done.
School grades don't work. Because people make comparisons between things that aren't really comparable, a lot of people get jobs when perhaps they shouldn't or don't when they should. This is much more like a system where traffic lights are sometimes all green than ones which are timed in an annoying way.
What's the right thing to do when traffic lights are malfunctioning this way? The first thing you should do, obviously, is call the authorities. But what if the authorities don't believe you? You tell them that the lights are all green, but they know their system works perfectly.
This is further complicated in the case of grading, because there isn't one central authority. There are thousands of schools and employers who all work with this system; getting them all to switch is no mean feat. So let's instead imagine that every traffic light is independently owned and operated, and that while the traffic light operators feel like they are all working together, there are some subtle differences between their algorithms which obviously lead to screwups.
Since the people who run the traffic lights aren't responding to you, what should you do? Obviously, for your own safety and the safety of those around you you should stop at every intersection regardless of the light color. And if you have the ability, it would probably be a good idea to climb up and cover up the lights yourself, so that others who come to the broken intersection don't get confused.