I have a prof who is changing the weight on my grade, due to doing so well on individual tests but terribly on group projects. My group is intolerably awful, like half never speak and none are prepared. In this case, i found it appropriate to ask. For doing crappy on your own part on some midterm, only if you had some documented illness on that day. In that case, you'd be better off no showing. What you're describing sounds like high school/jr college.