After receiving her paper from Professor Carlin, Sabrina says this is the first time she's ever gotten a C. However, in Season 1 Episode 22 (The Great Mistake), Mr. Pool gave Sabrina a C for her lackluster cell project, saying "This is not the kind of work I'm used to seeing from you. Look, you left out the Midrocondria, it's the power house of the cell."
When Sabrina brought her Professor the gift basket for a brief moment you can see over the set wall.
At 19:36, when Professor Carlan stands in his classroom, the top of the set is visible. You can see that the ceiling is unfinished.
Sabrina claims she never got a C before but in Junior year of high school in English, ironically, Sabrina got a C minus on her short story. Also in her Freshman year she got a C on science project.
In this episode Sabrina has a freak-out over a C and agrees that it is her first one but earlier in the show when she was in High School she got a C- in Season 3 Episode 18.