Freshman Trash Day/Senior Holiday Party: The last day of school before winter break was ALWAYS freshman trash day. Apparently in the past it had been worse, where seniors would tape the frosh into trash cans and roll them down the stairs, or things like that. The administration has sinced cracked down on it, and by my freshman year it consisted mainly of a few select freshmen getting duct taped to lockers and some of the rest getting "FROSH" written on their foreheads in permanent marker.
To crack down on trash day several years ago, the administration instituted an official "senior skip day" on what was traditionally freshman trash day. Somewhere along the line, this morphed into a day where seniors would sneak into the school very early in the morning the day of freshman trash day, decorate the main hallway outside of the cafeteria (Christmas trees being the primary source of decorations), and then spend half the morning partying. (Of course, many seniors came to school drunk that day.) The decorations grew more and more elaborate every year, and my freshman year it was rumored that the seniors had brought in over 150 Christmas trees and propped them up all over school.
After that year, the admin put a ban on the Christmas trees for two reasons: (1) they were a fire code violation, and (2) rumor had it that many of them were obtained illegally, including a group of senior boys who had gone out and sawed down evergreens off of golf courses! Of course, after that there was a push to make up for the lack of Christmas trees -- my junior year, the hall was filled with millions of foam packing peanuts. Another year, the seniors collected a pair of underwear from every senior and strung them up on clotheslines across the hallway.
Senior, Junior and Sophomore Walls: My school had these walls that were about three feet high that ran through the main hallway. Everybody hung out around them before and after school, sat at them at lunch, etc. One was designated for each grade (except the freshmen, who weren't special enough to have their own) and usually the most popular people from each grade hung out there. It wasn't strictly enforced or anything, though (if you were a sophomore and your boyfriend was a junior, you could still hang out at the junior wall).
Unofficial Senior Skip day: Yes, even though our principal handed us a free skip day in December, we went to school on the official skip day and waited til May to actually skip out of school. We were screwed up.
Upperclassmen also didn't have to go to school on the first day . . . that was for freshmen only, so they could have a chance to get used to the new school without all of us scary big kids there.