School day counts are established by each state, so the rules do vary. But most states have some minimum number of days. Depending on school contracts and state laws, schools may schedule a few extra days, to cover unexpected days off, Viral infection, Weather, or what ever. But if they fall below the minimum they have to add days. In Michigan schools can petition the state, for extenuating circumstances. I know sometimes if the limit is crossed near the end of the year they will get the waiver.

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Just Triing
Triathlete since 9:56:39 AM EST Aug 20, 2006.
Be kind English is my 2nd language. My primary language is Dave it's a unique evolution of English.

In GA, we had one or two built in snow days when I was a kid. Almost never used them and they became teacher workdays or something like that. They still have them built in as teacher workdays here but ever since we had the ice in 2014 and kids getting stuck in schools overnight due to traffic jams, they get panicked every time it gets cold. This year they've already had maybe 6-7 school days cancelled for weather. So to make up for it, they let parents vote on the solution. It was either cancel the week long winter break or add 30 minutes to each school day for the next 2 months. Parents voted on the 30 minute option - less disruptive for most.