As a huge fan of these books, I can help you here... BUT... I won't spoil it for you!
Mr. Brunner had the expectations that Percy Jackson should know the answers about the Greek gods and Latin, even when he didn't. He always looked disappointed when Percy didn't know the answer.
Percy though these expectations were unfair because he thinks that he is just a "stupid kid" and doesn't know anything, and can't be the perfect student. How wrong he is, I won't say.
Hope this helps! Have a great day!