I'm reasonably sure its got more to do with the fact that once you start getting fun things like free periods you don't really have a 'class' anymore. And as such its not appropriate to refer to them as a class. So the clear way is to start refering to chunks of time, which in this case is called a 'block,' and there you have it.
Now one can safely say that 1st block one is a teacher's pet, 2nd block one has a math class, 3rd block one has free and 4th & 5th block one has english class.
Neat how that works, huh?