The ruling comes after teachers at 13 Chicago schools have waived portions of their contract and voted to lengthen their school day.
The union's complaint now goes to the attorney general and a judge will decide whether to repeal the 90 extra minutes a day currently approved at 13 schools and implemented at nine.
A longer day is not in doubt at UIC College Prep because it's a charter school. Students receive two more hours of instruction time each day than those at most other Chicago Public Schools. Their ACT test results are four points higher than the city average.
"Our students actually get double math, double reading, and double English, double science in the four years they're with us," said Tressie Mcdonough, UIC College Prep principal.
"School board administrators and many principals engaged in a campaign of coercion and intimidation," said Robert Block, Chicago Teachers Union attorney.
Downtown, lawyers for the teachers union and school board were in a hearing room. In a unanimous vote, state labor board members agreed that the school by school roll-out of longer days violates the union contract and may need to be stopped.
"We would hope this would mean the board of education honors our contract as written and negotiate it with us. That's always been what this is about," said Karen Lewis, Chicago Teachers Union president.
"It would be a detriment to kids to not have it in the first place. To have it, take it away and put it back is even worse. I can tell you I have 400,000 more kids in need of it, and we're going to keep pushing for it," said Jean-Claude Brizard, Chicago Public Schools CEO.
Caught in the middle are students and their parents, on a day when a new analysis of test results found nearly half of Illinois public high school students failed reading, math and science portions of statewide exams.
Davor Engel has two sons at the Disney 2 Magnet school, one of 13 where longer days have already been approved.
"We have to do something. There's an argument that maybe tests are too hard. Well maybe the kids haven't learned enough," said Engel.
In the coming weeks the attorney general will decide whether to take the case to court and ask a judge for a quick ruling. That means the nearly 400,000 students who recently switched to longer days could go back to the shorter schedule for the final few months of the year, only to switch back to the longer day next year when state law allows the longer day at every school.