Teacher time is using the noun 'teacher' as an adjective. We use this construction to refer to the thing as a concept - in this case, like an activity. It's useful to have some teacher time in the beginning when you're learning something new. You could replace this with lesson time or something similar. Teacher's time uses the apostrophe to show possession - the time belongs to the teacher. Your teacher's time is valuable - don't waste it. [Don't waste the time of your teacher.] Your teacher time is valuable - don't waste it. [Don't waste your own time when you're in the lesson.]