Even when I was a small child (19 years ago now) Halloween costumes were not allowed, and it was understood that if you wore one to school, you would be sent to the office and your mother would be called and you would be asked to change clothes. Every school district I've been in has been like that. Their perspective is that something like Halloween takes place after school. Certainly, we did certain activities on Halloween that were Halloween-themed, but only a couple of classes I was in during elementary school had anything resembling a party.
Though one year our ASB managed to get around the no-costume rule because Halloween was during Homecoming Spirit Week, and we had dress-up days all week long anyways
I don't think it's the school's obligation to celebrate Halloween or any other holiday in any way, shape, or form. It just distracts from the purpose of school, which is education, not harvest parties, holiday parties, or anything of the like.