In university, homework is everything. If all you do is attend the lectures, you will fail the midterm and final. In order to do well in the course you need to do all the homework which really pushes you to learn. The examples given in lecture are completely trivial compared to the homework problems which can often take hours to solve.
“Enrichment activities” would actually be useful if the content was actually enriched. Advanced students would get a lot more out of school if they were given work that was genuinely challenging for them and took a long time to figure out.
My first collegiate math class, the teacher came in and asked the class, "do you know how to get an A in this class, two hours of math homework a night". I followed this advice and bam, A. Despite the fact that this math class was much harder than anything I did in high school and I was always a B+ math student.
The next semester I followed this advice again but let myself slip a few weeks and bam B+. The next semester I strictly spent the time again, bam, A.
Through upper class years my diligence in school was consistent but I sometimes had to focus on programming over math. I noticed a near perfect correlation between the time I spent on out of class work and my performance.
My grades weren't that great (classic B student) and I doubt I had better mastery of the material than the more studious ones in class. But writing the exam was both fun and challenging--I definitely had improved mastery afterward, as well as a newfound appreciation for his job. He gave me a B+, though, out of fairness as I had several days to write the exam, open book. Which was fine by me.
I think it is quite clear that being inately smart could cause quite a setback as one thinks they don’t need to work hard and winging it would do it. It turns into a bad habit and eventually one falls behind and stays there.