As someone who was briefly an art major, drawing is largely a matter of practice and having the proper foundation (which is gained by practice). Classes increase the rate that practice helps. If you practice, art education teachers have already figured out how to help you improve. What you see in high school art classes is the division between people who care and people who don't. People who don't care don't practice and consequently don't have the foundation to practice further. Because there's no future in art, in college the only people who take art classes are the ones who care and who have the proper foundation, so (allowing for the typical small class size) there is a much more normal distribution in skill.