The elementary school where I work has a costume parade up and down the halls. Parents show up with cameras and grandparents in tow and line the hallways to wave as junior walks by. Masks, scary face paint, and weapons (real or fake) are not allowed. After the parade, each classroom is allowed to hold a short Halloween party. By the morning recess at 9:20, all costumes are required to be removed and placed in backpacks. The rest of the day is relatively normal.
My classroom doesn't participate in the parade since we're a special needs preschool class. Walking in line is difficult for our kids and the crowd along the parade route is too overwhelming for them. Instead, we have them sit in chairs outside the classroom door so they can watch the big kids go by. We have one or two each year who get scared, so one of us will take them inside to play until it's over. After the parade is over, we take our kids for a little trick-or-treating walk. We leave bags of treats (non-candy) with our principal, office secretaries, speech therapist, and librarian. We take the kids around to collect the treats and show off their costumes to the adults on campus that they're familiar with. After that, we help them put their costumes away. If we leave them on all day, the energy level in the room can get pretty crazy. We still have to keep our schedule of speech, occupational therapy, etc., so we need to move on with the day.