To unlock the Referee! achievement in Little Nightmares 2 you need to equip the hat that looks like a football and run through the goalposts that can be found just before you climb the blanket rope that leads into the school. The hat can also be found on the dumpster that is next to the goalposts. Here is my own video guide that shows how to get the Referee achievement along with all of the other collectibles/achievements that Little Nightmares 2 has to offer. The Referee achievement guide is shown at around the 7:35 mark during the video.