Remember this rant about student participation in group discussions?
Clifton's post about the dangers of "group think" is spot on and analyzes the problem in more depth:
"Here's what happens in groups-if you're lucky. One student will take the assignment seriously, two or three will at least be interested enough in watching the serious student do her thing that they'll take part. The remaining four or five are grateful for an opportunity to cease all rational cognition (if they'd ever engaged in any in the first place). In the end, one student does the work of all seven, with minimal assistance of one or two others. And all get credit for the work of one."
I see a similar (although not quite as willful or jaded) dynamic occurring when I teach Latin. Even with only 11 students, the group structure allows students who are struggling to "slip through the cracks" far too easily. This is one reason why I give quizzes and tests often and base grades on a wide range of factors.
Class participation or "group projects" are usually poor indicators of actual learning and mastery.