I'm pretty confused when I first saw this term, because there is no such thing in Mandarin.
Then I did little research and figured out what it refers to.
The exam for college admissions in Hongkong (maybe the UK system, I don't know) consists of 10 subdivisions. Those outstanding students who get As for 7 sections are called 7A狀元 (usually 7A3B).
"straight A's students" is reasonable, but if you really want to specify the "7", you may need to give bit more background info to those who have no clue about the test system of HK.