If you want to horrify your school-age children, here's a great way to do that:

If you want to horrify your school-age children, here's a great way to do that:

Tell them that you think their school days should be longer. In fact, tell them you like the idea of school days that start at 8 a.m. and run to 5 p.m. — give them an hour or so for lunch — and let's even set aside a few hours for additional work on Saturday mornings.

Well, cue some horrified reactions: Five states have announced plans to add at least 300 hours of learning time to the calendar in some schools starting in 2013.

It will be a significant addition to the school year in those states — Colorado, Connecticut, Massachusetts, New York and Tennessee.

To see how significant, consider this: The state of Oregon requires that high school students get a minimum of 990 hours of instructional time in each school year. (The requirements are somewhat less for middle and elementary schools.)

The cost for the experiment in the five states giving it a shot will be paid by a mix of federal, state and district funds, with the Ford Foundation and the National Center on Time & Learning also chipping in resources.

The idea behind the experiment is fairly obvious: If you keep students in schools longer, they'll do better.

Education Secretary Arne Duncan, long an advocate for expanded school days, welcomed the experiment: "I'm convinced the kind of results we'll see over the next couple of years I think will compel the country to act in a very different way."

Well, maybe. We don't think the idea of additional school hours is necessarily a bad one, but we're not sure that it's a guarantee of higher performance for schools and students.

It's not always the number of hours in a school day but the quality of the instruction that's included in each hour that really can make the difference. If you're offering mediocre instruction in a seven-hour day, just adding another hour each day won't matter.

And that doesn't even get to the issue of how Oregon would pay for any kind of expanded school day — or even if this would be our first choice to improve our schools if we stumbled across some kind of windfall for education. In fact, many state school districts now are struggling to keep their doors open even five days a week, as witnessed by the number of districts experimenting with four-day weeks.

(Speaking of which, some districts are working to maximize the number of full weeks they offer on their schedules, with the idea being that shorter school weeks often wreak havoc with teachers' educational plans. It's a good initiative, considering how those shorter weeks often increase the burden on working parents.)

If the nation continues its experiment with longer school days, though, it might well be worth taking a look at restructuring or shortening summer vacation — after all, schools spend a lot of time bringing students up to speed after the lazy days of summer. And the need for lengthy summer vacations, frankly, is a relic of those days when all hands were required to labor on the nation's farms.

Shorter summer vacations? At the least, that's the kind of proposal that could elicit a whole new batch of horrified reactions from students.