Brennan protested, "At least I'm learning magic now! I'll be way better than you 'cause I'm older." He was trying really hard to learn magic because he had to catch up with the other students. They were so much older than him, that he was left behind. It had to be because they were older, not because they were better.
At least, that was the idea that he clung to in order to keep going. He would beat them, but first he would have to beat Imenry! It was hard to scare Brennan away, because the scariest person that he knew was actually his mother. At least, some of the time she could get a bit scary, but she was also his mother. That meant, to his childish mind, that even scary people are not scary all of the time. So when he is ever scared of someone, he just waits and comes back when they are less scary. Unfortunately, that does not always work because the head mage was usually scary. It seemed to be his default, at least in how he taught Brennan.
Brennan was not old enough to realize that simply because he found the man low level scary did not mean he was actually trying to scary and was instead just a stern person.