Well there's something not true about this, sally is very much in the same science class as MC.
While it's true she neither in her English or Maths, there's nothing to suggest she's not in another class and was therefore taking the exams too.
I mean it was common when I was at college that there were I think 3, but might have been 2 maths classes at the same time. It's a stretch I know. And probably not the case here.
What we know is their dialog :
mc "What are you doing here?"
sy "What does it look like I'm doing here?"
mc "This is an intro class. Did you flunk it in the past?"
sy "Euw! I do not flunk courses. Just because that's your frame of reference to college doesn't mean it's my reason for attendance."
mc "Then why are you here?"
sy "I need the STEM credits, ok?"
mc "Ah! And this is an easy course to get that. Gotcha."
sy "Easy course, my ass! I may lack the prerequisites for a more advanced class, but that doesn't make this class easy."
mc "I'm confused. You're saying classes are easy since you don't flunk them, but that this class is hard for you?"
mc "Is the class easy or hard, Sally? Make up your mind."
sy "Bite me!"