At my elementary school this was actually encouraged for students excelling in maths[1]. It was offered - but not stressed as important whatsoever - that if you completed the official book of problems ahead of time, you could begin working on the extra book(s) named "Mathimagination". If my memory serves me, there was no extra credit afforded for taking this route; rather, it was purely to keep students interested in and progressing with a subject they enjoyed and in which they excelled. I credit my teacher that year for getting me excited about mathematics, which carried on for many years.
On a related note, my high school had an Advanced Placement English class offered to students selected by previous semesters' English teachers. This option came with an unfortunate snag for all students enrolled in the French immersion track: both the AP English class and a required French class occupied the same period. We were frankly offered the choice to stick with the French immersion we'd been part of for more than 10 years (having started in kindergarten), or to convert to the English track by dropping all French-language classes entirely.
Yeah, the AP class that year was quite small with not a single person dropping the French track. It turns out that people enrolled in the French track are much more likely to land placement in AP English, as being fluent in more than one language steers a person into understanding and appreciating languages more than someone immersed in a single language. Such a ridiculous scheduling blunder by the administration; just the memory of not being part of that class more than 10 years ago makes me sad.
[1] Having grown up in North America where it is common to refer to mathematics as the singular "math", it is still weird to type "maths" even after having picked up the habit a few years ago.