Okay, some courses aren't available to people who have a credit for something else in high school
(e.g. if you took Gr12 Data Management in High School, you can't take Stats 1st year)

Does the Gr 12 credit count if the course was taken in summer school?

(and yeah, this is probably I should've asked months ago... but even then I *still* think it's a stupid question but I can get overly paranoid over the smallest things and I'd probably lose sleep over this over the "what ifs" subconsciously...)

As far as I know summer school grades don't count but you should talk to the registrar's office about it.

As far as the Ministry of Education is concerned, a credit is a credit, no matter when it's obtained (regular school year or the summer). I would imagine that Mac sees it the same way - so if you took the course in summer school and it's an antirequisite for a course at Mac, you probably can't take the course at Mac.

(Unless you're referring to your actual mark in the summer school course, in which case Sabrina's answer is right, as far as I know.)