Once Adar itself hit, the pranks and shtick would start. Seniors usually led the way, but every grade was doing stuff, while the Beis Medrash guys would be putting up hilarious signs and having an "election" for Purim Rav, replete with campaign stickers, ads, buttons, and the like. The pranks would sometimes lean into the extreme, and the most commonly used prop to add a little life to any good idea was to insert dummies. This was usually incredible - WITS is a huge old mansion on the shores of Lake Michigan (bought for $600,000 when it was worth ~$1.5m at the time) with a grand entrance hall, so there's a nice area to set up themes. When I was a freshman, the seniors set up a full-scale bar, complete with a drunk and a bartender. As a sophomore, the seniors built a jail cell, including a guy pleading with his hands stretched through the bars, a guy sleeping, and a guy throwing up into a toilet (which they'd inserted into the cell). Once in a while, the dummies would go a bit too far; the class that hung one out a front window to be Haman resulted in cops showing up, as people driving by thought they saw something a little less innocent.
Rosh Chodesh, though, was typically the beginning, and was a smaller prank. Taking the whole school's shoes and spelling out "100" in a construction area where $100 fines were assessed for going, spelling out Adar in the main hall... cute but nothing too special. This year, it appears that the seniors at WITS went all out on cuteness, buying up a bunch of baby chicks and then, during breakfast, letting them roam free. Later, they gathered them up* and brought them to the science lab, learned a little about how to take care of them (positioning lights to keep them warm, etc.), before sending them at the end of the day to the Wisconsin Aviary. Here's a video with a few pictures and a few seconds of video:
* Whenever I think of animals roaming free during a prank I think of the classic prank letting pigs/cows/sheep numbered 1, 2, and 4 roam free through a school and watching the school gather them up.