When I was in Chicago public schools, 70% was maybe a D. 50% would be failing in any class outside of some hard college courses with strong mercy curves. (Perhaps the teacher wanted to give room to excel, or had no idea how to write a good test to the material... These were all advanced math courses where you had to do more than regurgitate knowledge.)
30% seems pretty low. Like you don't know 70% of the material but still pass?
Well, at least in Uni..."passing" alone doesn't mean much, there's still a cumulative GPA to maintain, and if you only passed all courses, you're still getting the boot (or another scenario- get an A+ on a course to cover for another you got a D in).