Take this with a grain of salt as I am a college student with no real work experience or "colleagues". But I agree with MSPEconomist, will it change your work relationship? When I fly out of college with friends, I always give up my first class(either paid for by my parents or comp upgrade) seat to sit with my friend, because my parents told me that it is rude otherwise. I think that your offer to get C+ for them was kind, but may put you in an awkward situation if they were thinking you would be sitting with them?