Chiliverde, I think the consensus is that if it is a big part of their marketing, vs a slide show on the site that comes and goes, and if it seems to be a major hoopla in publication and promotion... you might want to avoid that school. But having it on the calendar - that's not a deal breaker.
I would posit that a photo or so on a website is also different from having it as a major part of the marketing. Check also that you aren't looking at a local holiday/tradition (Dia de los Muertos, Pangangaluluwa, Fasching) or possibly a school play or curriculum-related event that involves dressing up. Basically I agree that it shouldn't be a major part of marketing, but a photo of kids dressed up is not a turn-off in and of itself. It's more about if they have nothing else to show.
I've heard this--at the elite school I worked at there was no Halloween celebrations at school and at the next school (which was not good) Halloween was huge. At my current school, it's very good, but we do go all out on the day, lots of photos on social media of that day on that day--then it's back to business as usual, for what it's worth.