Online course assignments, social media, adapting curriculum, concern about those students whose motivational/attentional issues make it much harder for them to participate. It's all there.
An elementary school contemplated it, 500 families supporting, 100 threatening to leave. Major complaint: different calendar for elementary vs. junior high/high school.
Clark County is contemplating year round, despite high cost of AC during Nevada summers, with a complex tracking system (6 weeks on, 2 off, if I understood it correctly), with a goal of reducing overcrowding/improving capacity utilization. Schools, they really are factories for learning. ;-)
Wake County already has a similar set up in place, for similar reasons.
Rosendale-Brandon was considering year round to deal with summer learning loss, but tabled it after they saw the state's proposed budget and decided they couldn't afford to experiment.
Interested in opinion and experience, if anyone has any. This has all the indications of one of those topics that people have generally not thought about, and then as soon as they do, the lack of concrete answers to important questions results in a lot of FUD.