How do universities look at my grade 12 marks?

Well the university of your choosing will, depending on the faculty you've applied to, look at your top six 12 U marks. If the program requires specific courses then they will look at those marks - there are usually two or three spaces for your, hopefully higher, marks to fill. Like, for example: engineering, it requires english (like all courses), calculus, advanced functions, chemistry, physics and one other all at the 4U or 4M level.

Although I believe I know that I haven't quite answered you yet. If you have not completed a full 4U, or grade 12 course yet they will base your marks on grade 11 and your mid-terms while examining you for admition. As the year moves forward they will look at your completed 4U courses from the first semester and the mid-terms of your second semester. While doing so they will focus on the required courses. They can and will access your records to get a better picture of the type of student you are. But seeing as you're worried about it now you're probably in good shape.