I've added a feature request to Moodle Tracker asking for basically this feature:
Here's the text of the request:
"Adding leveled completion options to activity completion would allow teachers to track progress toward a goal rather than simply log that it's been completed. It would also augment their ability to award badges for course participation. For example:
I want to create a forum where a user is given badges for participation in a forum as follows:
10 posts = Little League Forum Poster Badge
20 posts = Minor League Forum Poster Badge
40 posts = Major League Forum Poster Badge
As it is currently set up, I can only award a badge for activity completion, which is restricted to one completion per activity. This improvement would allow me to award a "level 1 complete when condition A is met," a "level 2 complete when condition B is met," "level 3 complete when condition C is met," and so on, such that users could be awarded badges (or anything else that's being tracked through completion) progressively as they continually participate in an activity."
It might be worth it to add grade level as a completion option, since that would match most closely to what you are trying to do.