11-year-old Math Class

[rollerramla]

No: HPFGUIDX 35635

This is a question I've had for a very long time; in fact, from the 
moment I started the books I wondered this. If the students enter 
Hogwarts when they are 11 years old, then does that mean that they
stop 
learning math at a 5th grade (or the British equivalent) level? If
so, 
it seems that the students graduate having only learned simple 
arithmetic! Any theories? I'd love to hear them. :)