@Kingdo
I'm glad he isn't getting special treatment just because he's the MC. Letting him off the hook because he didn't know any better would mean that other people would be able to plead ignorance as well. The rule is probably in place so they can warn others to stay away from an area where people are practicing high level magic.
I would have liked to see the girl also given a fine because she was the one to tell him to keep going after the first spell. I'm sure if they had immediately gone back and reported the first spell as being a lot more powerful than they were expecting, then there wouldn't have been any issues.
She's obviously aware of the rule since she came in to pay the fine for the MC, but still had him continue.