Ranked by total GPA, The [ noun here] of our school is Jack.
Does "forerunner" or "first-ranker" work?
forerunner means "someone or something who came earlier than something else". That's not what we mean. You're probably thinking of frontrunner.
first-ranker is understandable but not idiomatic. First-ranking is acceptable as an adjective, but I don't think I have ever heard the noun first-ranker used.
The highest-ranking student in a class is usually called the valedictorian, if you're looking for something specifically applicable to students.
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.