I'm glad I took AP instead of IB. I go to a very difficult magnet high school. I was hovering around a low B for my entire AP Calculus BC class, but I still flew through the AP test and got a 5 (highest score). Even kids who get Cs in the class get 5s on the AP exam. I'd imagine that there's a number of similar schools with a large IB program and highly deflated grades, and I doubt that the IB algorithm takes school difficulty into account.