I'm going and I go are not the same in Japanese.

Ikimasu generally translates as "I'm going somewhere", kimasu more as " I go or I come ".

Now, going is an action that either hasn't happen yet or is an action that is currently happening. Whereas kimasu will be used once your action is completed. Kimashita will be used to refer this action when it has come to past, so when your teacher leaves school.

Makes senses? cause I can try explaining differently if needed :)