Quite possibly the dumbest thing I've ever heard.Dunno. in Ruse, the marks are REALLY tight, so they can't do it based on marks, but rather on ranks. And you do get ranked by surname. So if your sruname starts with Z (common), and you get the same as someone whose surname starts with Y (also common), they get ranked higher than you. But essentially, top 90 means you're getting at least 90% on your 2U prelim maths paper and at least 70% on your 3U maths paper. For English it means you're getting 19/20 consistently in English in a variety of assessments.
And on topic, drop chem since you said you don't really like it