In our region, we call this person the topper of the school/examinations/etc. Being a topper implies that you're the one who got the highest GPA in the entire class/school/region. It has nothing to do with valedictorian/farewell functions.
Moreover, as opposed to what J.R. pointed out in another comment, you can be a topper at any time around the year. You can be a topper in the monthly tests, or in the half yearly exams, well before the time the class will graduate.
It does have a Google Dictionary entry for it:
an exceptionally good person or thing. (British, informal, dated)
While it doesn't refer to marks specifically, you'll surely get the idea of this context if you just google "cbse india topper" - for an idea of the high frequency with which it's used in India.