Hi Sean,
Unfortunately, no, there isn't a single call to get grades for all the students in a course at this time. We implemented a grades module for our CampusLife offering, and in fact discovered that there isn't a way to get all of the grades for a student with a single call: a list of grade objects needs to be requested, and then grade values for each object need to be retrieved separately. This is a known limitation with our API but I don't have any insight into plans that have been made to address it.
Thanks,
Mike