What do you call the best pupil in a class in a single word?

Not a single word, but how about, "the star student"? A "star" in this context is recognized as excelling in the academic sense. And by using "the" it indicates that it is the one person with that designation.

The best match I can think of is the term "valedictorian." This word is only appropriate in the context of a graduating class, but it is the closet single word match I can think of.

According to Merriam-Webster the definition of valedictorian is:

valedictorian - noun - the student who has the highest grades in a graduating class and who gives a speech at graduation ceremonies

It depends a lot on the definition of "the best"...

• The best at extra-curricular activities
• the student exhibiting the best behavior
• the best at sports
• the best because of highest grades

....if the pupil is good at most activities, he/she is an All-rounder.