Will High School expansion pack have grades?

They never mentioned it, but if I had to guess, I'd say no. I'd say in the story they created they were thinking all teens would be graduating, and since the focus of the pack wasn't really the "school" but more about teen life, I don't think the actual schooling system is too in depth.

I don't think grade levels will be there as players have varying control over sims' aging. They can even stop sim's aging after all! Or age them up early with a birthday cake.