I got good grades at school and am now getting them at a decent uni in a mathematical subject.
I reckon school is equally or more academically challenging and requiring of intelligence. But uni is proving more tough for me overall. The biggest thing which is hurting my chances of a good degree grade is organization, and handling all the spare time (few lectures every day and not that much stuff to learn and do for the course). Most people can have organization. It's just a massive pain. I've missed exams worth big chunks of my grade because the exams can happen pretty much anywhere in my university city, the uni property being so spread out, making it difficult to get to the right place at the right time.
There are big penalties for missing deadlines. And they offer help classes for projects we have to do, but I don't have the organization and motivation to work out where and when these classes are happening. I get through the projects myself (mostly), but it's a lot more work than it must be for the people who go to the classes where they're apparently pretty much given the answers.
Also a degree lasts a long time and it's easy to get unhappy if you've just moved away from your nice parents' house with warm company in a nice area to a manky student flat full of ********s in a horrible area, and aren't having a good time. There are loads more people in a uni, so you can be treated like a nobody which can make your unhappiness worse. So how well you do can eventually become more of a question of your motivation and happiness than anything else.
So if you did well at school, I don't think you're stupid