In America, most places is five years in elementary, three in middle, and four in high school. You'd think it'd be the most sad leaving elementary, but it isn't the "being around them for so long and then leaving" that's the saddest. Putting aside for a moment that you will likely see a lot of the people again in the next school, the end of high school is still far worse because it's the end of an entire chapter of your life. People change, separate, and things are just not the same. Ever again.