Every time I've seen "year round school" proposed, nationally or locally, the premise has been that instead of having the block of 2.5 summer months off, instead, they take that time, and spread it out throughout the year, such that kids go for a period of time, usually 6 or 8 weeks, then they have two weeks off, then go for 6-8 weeks, then off for two weeks. This is how, from my experience, the proposals have functioned.
Here's the NEA's website on the subject
http://www.nea.org/tools/17057.htm
See?
This format is the type typically proposed. That's why it would be the alternative chosen, because its the one the NEA and these groups who want year round schooling have been pushing for well into two decades now, and I've never seen a different alternative proposed, and I'm not even sure what that would be. You have to understand, too, that teachers love teaching in part because they only "work" 9 months out of the year. The teachers unions of course see this as a direct benefit for their members, and they aren't going to go along with a plan that takes away summer without retaining the same amount of time off for teachers, just spread throughout the year. They would basically consider that to be being forced to work many more hours, for no extra pay (teachers are salaried positions) so yeah. The Chicago school district unions and employees almost rioted last year because the district wanted to make the school day one hour longer without increasing teacher pay. Just think about how the national union members would riot if they found out they were losing their summers and not getting the same amount of time off spread throughout the year.