For me it was usually "when the teacher explains it once", occasionally "before you ever cover it in class" if it was a subject I was interested in beforehand. I rarely did homework assignments and never took notes.

There's also an interesting thing you can do with standardized tests, where you infer the answer from the questions and the other answers given. Standardized test authors are actually really predictable: they usually think of multiple choice options based on common error patterns, so if you look at the answers and think "Well, those two are off by one, and those two have factors that are off by one, and those two are what you get if you add instead of multiply", you can triangulate between them and come up with the right answer even if you don't understand the question.

Actually, I once did an experiment to try and quantify this effect. Throughout high school, a bunch of my classmates would say, "You're not really smart, you're just a good test taker." So I figured I'd take the AP Comparative Government without ever having taken the class or done any self-study. Just a half hour of reading a test-prep book over breakfast. I knew I didn't know anything about the subject, so any score I got could be solely attributable to test-taking skills.

I got a 3 on it, which is a passing grade for many colleges. I got a 5 (the highest) on the tests that I actually did know the subject area, so it wasn't all test-taking.

It was funny though - the AP Comp Gov is a big multiple-choice section followed by 4 essay tests, and I obviously didn't know the subject, so I'd have to bullshit the essays. So I did the multiple choice through a combination of looking for patterns in the answers and using my common-sense knowledge of foreign governments that I'd picked up from newspapers. And then I used the questions they'd asked in the multiple choice section as talking points for the essay. Basically, I was using the test itself to answer the questions on the test.

People in school would comment on how well I did on all the various standardized tests, and my response was "It's easy, the answer is always right there on the page."

For me it depended on the subject.

Subjects like Maths and ICT were largely logical, I could work out most things without needing to learn them. For maths I would just read ahead in the textbook, anything I couldn't already have guessed I'd understand straight away, while classmates would need the teacher to explain the various concepts.

Languages (for me French and Latin) came easily - French was helped by the fact that I've been to france 30+ times in my life, but Latin was just as easy. Just remembered everything I learned (most forgotten now,

Subjects like English and History, I was a very fast reader, so instead of reading along with the class I'd just read to the end of the textbook/novel/whatever, and I found it all pretty easy. I never produced amazing insight into anything, but I could analyse anything to an ok standard without any effort.

The three science subjects, I found that doing really well on basic logic/maths/etc. and pretty badly on anything that needed learning averaged out - less so in Biology, though. But I have a pretty good memory when I try and apply it, so again I pretty much didn't forget anything that I'd read once.

As a broad non-subject based answer: I never bothered to read over notes just before an exam. Much of what we covered I understood before we got to it in class, and 99% of the time I understood it in class before the teacher got to explaining it. Homework was for me a "do as little as you can" thing, I'd generally either not bother handing it in or I'd do it during registration in my classroom at the start of the day I had to give it in. I guess the biggest thing school actually taught me was how to push the limits and know just exactly how much I could get away with. I managed to break more rules than any of my classmates (both in terms of work done, and other school rules) and yet never found myself in detention. That kind of stuff was actually what kept me slightly interested, for example working out the best ways to really annoy teachers without giving them justification to punish me.

The real TLDR answer is: Different children have different skills and different levels of knowledge. Education is designed not to leave anyone behind, so it aims at the lowest common denominator. Hence there are those who would be suited to higher levels of education than people of the same age, but the school doesn't cater for them.