They're the grades used in Harry Potter:
HIgh to low
O - Outstanding
E or EE- Exceeds Expectations
A - Acceptable
P- Poor
D - Dreadful
T - Troll
You will usually get an O for a lesson, though you can get an E or A depending on how well your character does / how long your post is in some classes. If you don't do a class, you automatically get a P. This isn't the end of the world, you get an average grade at the end of a semester out of all your grades.
So if you do a few lessons that get an O, but also don't do a few other lessons of the same subject. You'll probably end up with an A or E average at the end of the year.
Also, you don't tend to get lower grades (D or T) unless you ask for it specifically I believe.
Hopefully...