Why "the" in "teach the 3rd grade"?

When asked "What do you do?", I understand it’s idiomatic to say both:

1. "I teach the 3rd grade"
2. "I teach 3rd grade"

Note that the former can be used even as a general answer, not just as meaning "I teach the 3rd grade of school X". In that case, what does the "the" mean?

Inspired by Why "the" in "looking in the mirror"?