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The next few days are critical for many schools. It's the time of the year when attendance has a major effect on a school district's bottom line. Take Detroit. On Wednesday, students who show up could win a bike or an iPad. - 

For one day only -- this Wednesday -- elementary school students in Detroit will get entered into a drawing for a bike (at least the ones who show up and stay for lunch). High school students will be eligible for iPad Minis. It’s worth it for Detroit because every kid in school that day is potentially worth a bunch of money.  

States distribute around 40 percent of school funding, usually on a per-pupil basis. So, how do you count the pupils? The simplest way is to take a snapshot of how many kids are in school on a given day: Count day. Around a dozen states do things this way, and some would say there are downsides.

"That creates a kind of perverse incentive for schools to try to make sure they get as many bodies in the classroom on that single day, as they can," says Thomas Dee, an education professor at Stanford University.

In other words, focus only on that day, bring out the iPads, and never mind about the rest of the year. 

Detroit Assistant Superintendent Steve Wasko notes that Michigan’s system isn’t strictly a one-day deal -- there are nuances -- and downplays the importance of these incentives.  

"We don’t really believe that a parent is going to decide to send a student to school  on a particular day given that an iPad is going to be raffled off or not," he says.

But he hopes it won’t hurt either. 

Follow Dan Weissmann at @danweissmann