My kids go to a public Montessori school. Each grade dresses up but in a theme that goes along with what they are learning. I would prefer it if they could just do the regular costume, but the kids don't mind (to them its an extra costume) and I LOVE pretty much everything else about the school, so I'm not going to battle this. They still get parties and a fun day.
Kindergarten theme is community helpers. Ds is going as a scientist by day Jack Skellington by night. 2nd Grade is a Pioneer theme. They are going on a field trip to a museum where we will spend the day as if we are Pioneers from the 1880's. So dd, dh and I will all be doing our best Little House on The Prairie tomorrow and then at night dd will get "the best of both worlds" by dressing as Hannah Montana.