Does "in charge" here mean "I'll have an absolute control over them" because there are only 17 of them?-- Well, not 'absolute', but the teacher intends to maintain control of the class.
Does "go running down a mile of corridor" just mean that she won't run fast along the corridor to another class?- It means that she won't have to change classrooms.
Ok, but to be more precise I think it says that when the bell rings she won't have to leave her classroom and run along the school coridor to another classroom.
I was juct curious about the phrase "a mile of corridor"? Probably it means a mile long corridor.