"Classes and Training" can be found either in Hogwarts or in Ottery St. Catchpole. You will be given the option of either questioning your professor or challenging your professor. Questioning your professor allows you to see exactly how much experience you still need before you are able to level up (without doing the math in your head... or a calculator). Once you have enough experience (the experience bar will turn blue) you can choose "Challenge Professor". You will not retain your buffs or be able to use your special skills during this fight (though it is returned to you afterwards). Professors are tougher than the things that you will fight in the Forbidden Forest.