Geez. That was an example.
The split before the season is more "fair". For argument sake, let's say schools have between 1 and 1000 students. If you split them up just for playoffs, that means that you would have classifications from 1-250, 251-500, 501-750, 751-1000. All the schools that are in 1-250 will have to fight for playoffs. This would NORMALLY give the advantage to the schools that have 250 kids. The less kids you have, the less you have to choose from, the less play football, so you are NORMALLY at a disadvantage. Now if you split it up before the season starts, you now have 1-125, 126-250, 251-350, etc.
If you cannot understand that, I can't help you.