does the teacher teach one class or many that the student attends? Sort of depends on the meaning intended for the word class: class can mean a session (a class at a particular time on a particular day) or a course of sessions (geography class is every Thursday at 2 PM). So, do you mean each session, so classes, or do you mean course, and if so, does that teacher teach only the one course (class) or several courses (say, math and geography on different days?).
The expected usage form context is singular class, as in generic teaching sessions, rather than specific sessions, or multiple distinct sessions representing unique courses.