The teachers union is accusing Chicago Public Schools of bribing teachers at three schools that agreed to adopt a longer school day.

The teachers voted Friday morning to approve a waiver to their contract that adds 90 minutes to the day. Individual schools can move to a longer day if more than 50 percent of teachers sign waivers.

District spokeswoman Becky Carroll says it was the teachers’ idea.

“This is something they wanted to do, regardless of receiving any incentives or assistance,” said Carroll.

Carroll says teachers will get a lump sum of $1,250 each. That’s just under 2 percent of the average teacher’s salary.

In addition, two schools starting their longer day next week will get $150,000. Carroll says the money can be used on anything the schools deem is needed “to help ensure a smooth transition to a longer school day.” According to Carroll, that could include anything from teacher aides to technology.

The union had advised teachers not to vote in favor of any waiver to their contract, but the 90-minute additions were approved at Melody Elementary on the West Side, Skinner North in Old Town, and the new STEM magnet school on the Near West Side.  Skinner and STEM will begin their longer day immediately. Melody will start in January. They join about a dozen schools that already have an extended day.

The Chicago Teachers Union immediately filed a grievance. The union says teachers are being coerced into holding the school-by-school votes to add the 90 minutes, and the payments amount to bribery.

The school district and the teachers union have been in a protracted battle over the length of the school day. The CTU rejected an offer last week to extend the day at all elementary schools in exchange for a 2 percent raise.

Schools chief Jean-Claude Brizard praised the teachers who broke ranks with their union as “courageous” for putting children first.