I've hated Halloween ever since I was a kid. I always thought it was the absolute worst day of the school year having to suffer through it. My only beef with it was flat out disinterest. I pretty much felt that way as a teacher too. I'm so happy not to have to do that this year since I'm retired! Nothing to do with religion or anything else, I just didn't like how people acted and it just didn't "fit" with school IMO. We always "did" Halloween in my classroom, but it wasn't over the top. I always discussed how some people "do" Halloween and others don't and that's okay and the kids knew that whether they dressed up or not was okay too. I preferred fall type activities for celebrating, but it wasn't "anti Halloween" it was just about a season not a day. The only Holiday we celebrated at school that was more about a day than a season was Valentine's Day and even that we turned into a wider celebration of friendship etc. As a teacher, I always felt it was my job to reach the kids who were "disenfranchised" from any holiday due to poverty, neglect, disinterest, or disbelief and make sure they still felt part of our community during those seasons.
That said, on Halloween there were always kids in tears because they didn't have a costume, etc. It's a whole lot of drama and just another pointing out of the haves and have nots at school that I would rather avoid. In my school (with a very nonreligious staff) the majority of teachers wanted to minimize school wide Halloween celebrations and have the big events happen outside of school hours. Teachers came out on their own time to support the carnival though.