Well, to be quite honest, we really don't know much about the school system.

The biggest problem is that Ashford is a private academy. We have no idea if there even IS a general public school system. And if there is, we have no idea if it extends to the Areas. And even if it does, we have no idea if the systems would be the same or similar.

But we do know that Suzaku is Lelouch's grade level, and instead of going to school, he had joined the military. This suggests that:

1. At least the last two years of high school aren't compulsory,
   
2. There is no public school system,
   
3. The school systems for Britannians and Numbers differ, or
   
4. Some combination of the above.