As a teacher, I wouldn't like that as much with kids coming and going depending
on the track, unless of course each class had its own track. But even though I enjoy having fewer than 180 school days and summers off personally, I truly believe it would be better for the kids to spread it out over 200 days.
If I were to start a school, I would have 200 days with the Singapore calendar, would make the school day longer, and have no homework. At my school we assign a lot of homework (75 minutes per night for the "average" fifth grade student). I say let them be kids when they get home, free their parents from homework stress, and give me more time to work with them and really get them where they need to be. I could go deeper into concepts at the same time as really working with them on their executive functioning skills.