It's clear from the name what this method does, but it would be helpful to include a doc string that explains the return values: 

- success (whether the grading service was contacted - line 321)
- allowed_to_submit (if the number submissions they have graded is greater than the number required or there are no more submissions available to grade)
- error_message (a student facing message to display if the graders aren't available or if they're not allowed to submit, a message describing what they need to do to get feedback.)