This is a tough one to answer because there are so many things to consider when creating. My most valuable lesson learned in art school could be about color, line, composition, value hence they are all important. I will say that perspective is probably one of the best things to know, because being able to understand this will make it easier to bend it appropriately. The same as understanding the measurements or proportions in portraiture. These are important to understand how to use and apply it to whatever your piece is. Turning a piece upside down, walking away to look at it from a distance gives you a view that may show how it needs to be corrected or tweaked. These are all different perspectives. Learning 2 point perspective and more is very helpful.
Gosh, this is a hard decision to say what THE most valuable lesson learned would be. Maybe it is something I learned from Stephen King. To be fearless! I think the ability to be fearless in your creativity is probably the most important lesson. Learn and be fearless!
Experimenting and seeing are also important as well as the love of learning that has helped me to continue to learn. A valuable lesson learned was to read, read everything I could get my hands on about a topic I was interested in. It is said that if we all learn to speed read we would indeed be very healthy, wealthy and wise. Just imagine that you read all of the Bible, everything about investing, everything about cooking, cleaning productivity, building anything… I am of the mind that knowledge is power. What to do with that power is another question altogether.