You must have been a fairly effective student, at some point in your life, to be able to write as well as you do. An average student with good study habits is very likely to go further in life than a gifted, but lazy, student who relies on what can be effortlessly retained. Also, there is no such thing as being so smart that one never has to study.

When you read, use a highlighter. Also, use a pen to write in the margins, when you think of a quick way of summing up a long or complicated idea. Also, have a notebook that you put down your interpretation of what you read that seems important.

Don't be a slave to the teacher or the authors of your books. Feel free to say, "I don't really care about this and I'm only going to expend so much effort on it." or: "This idea is meaningful to me, and I'm going to think about this for awhile."

I don't know that joining study groups is all that helpful. Basically, studying is a solitary activity. I would say: don't spend too much time on stuff that totally bores the crap out of you. Notice what arouses your interest and spend more time on that. If nothing in books interests you, then accept that you might do better to learn something more concrete, like a trade.

The only way to learn math is to do problems. Nobody understands it just by reading it, except for pure geniuses.

For other subjects, one thorough reading should be enough. After you read a section. ask yourself: "What was that basically about, and why would anyone care?"

Students waste a lot of effort trying to anticipate what the teacher might want them to know. Start forming your own opinions about what you think is worth knowing and why. While I am not a fan of "study groups," I am a big fan of discussing ideas with other people. A lot of that goes on right here at PC.