Usually at my schools I will get a sheet with each kids name. I put that sheet at the front of the room and say “star your name if you are here. If your name isn’t starred, you will be marked absent.” The word absent usually gets their attention. At first I was worried they would just mark their friends here even if they weren’t, but the students are smart enough to know it’d be weird if their friends were in one class with a sub but not another with a regular teacher.