GIVE ANSWERS - English

I'm reading Harry Potter and the Philosopher's Stone, the UK edition. As a ESL learner, I sometimes find it hard to understand some words and phrases. Of course I also read the translated version and I have different ideas.

Today I read "Professor McGonagall was again different. Harry had been quite right to think she wasn't a teacher to cross. Strict and clever, she gave them a talking-to the moment they had sat down in her first class." So how to understand "she wasn't a teacher to cross"?