In October, two unions representing 32,500 teachers and school support staffers walked out of Chicago Public Schools on strike — but 582 members walked right back in.

During the strike, which spanned 11 school days, union staffers at 187 schools and 16 CPS departments crossed picket lines to clock into work at least once, records obtained by the Chicago Sun-Times show.

The Chicago Teachers Union, which at times has been quick to denounce dissenters, says it won’t go looking to find strikebreakers but will hold hearings on possible punishments if told who crossed picket lines.

SEIU Local 73, the union that represents most school support staffers, says it isn’t planning to punish anyone.

For the CTU, an average of 93 of the union’s 25,000 members went to work each day of the strike — less than 1% of its membership.

The most CTU members — 138 — went to work on Oct. 31, the last day of the strike, when all issues but the number of makeup days had been settled.

That’s fewer than the 250 CTU workers paid by CPS during a single-day strike on April 1, 2016.

SEIU 73 reached agreement with CPS on a deal after seven days. But SEIU leaders urged their members to stay on picket lines with teachers as their colleagues’ talks stretched into another week.

For each of those first seven days, an average of 207 of SEIU’s 7,500 members clocked into work, a number that rose to 217 for the two extra days they stood with the CTU. That’s just under 3% of their membership.

SEIU has less than a third the number of members the CTU does but had more than double the number of picket-crossers, most of them special education classroom aides and security officers. Union officials say that’s because SEIU members are among the school system’s lowest paid workers.

The constitutional bylaws of both unions allow them to punish strikebreakers.

CTU leaders say they won’t use CPS data to find those members and instead will rely on reports from colleagues about strikebreakers, who will be called to a hearing before the union’s executive board, CTU organizer Matthew Luskin said. They could then lose union voting privileges or have to pay the union any wages they earned during the strike to hold onto those rights.

SEIU 73 president Dian Palmer said that many of its members who work for CPS in low-paid positions couldn’t afford the hit on their wallets they would have taken by staying off the job.

“As a group, with our workers who were making $20,000-something a year, we just could not hold them to the same standard as CTU,” Palmer said. “A lot of people just could not afford to be off because, quite frankly, the bank doesn’t want to hear it. They don’t want to hear why you don’t have it. They just want you to have it when it’s your mortgage payment or your rent.”

Altogether, 201 members of the two unions worked for one day of the strike. Forty-one CTU members swiped into work on all 11 strike days. Twenty-one SEIU members worked seven of theirs. Another 166 showed up on weekends, meaning they could out-earn their colleagues this year by getting paid during the strike and working the five makeup days.

During the strike, CPS spokesman Michael Passman would not release figures on how many union members were showing up for work at schools and at the school system’s administrative offices. Schools officials released this data showing which union members came to work only after the Sun-Times requested the information under the state’s public records law.

CTU officials said the number of strikebreakers they had is even smaller than the data show because about 100 of the 208 people identified as having shown up for work during the strike aren’t CTU members — they either haven’t signed union cards or they opted out of union membership under the Supreme Court’s 2018 Janus ruling letting public-sector union members skip out on dues. Luskin said another group filled jobs — some college and career support staff, for example, who weren’t recognized as union positions until the strike was underway.

“These numbers are really small,” said Bob Bruno, a labor professor at the University of Illinois at Urbana-Champaign and a CPS-CTU expert. “The fact that you had such an insignificant number of people, especially considering an 11-day strike, it’s remarkable the level of unity and support that they managed to develop and hold onto.”

A couple of dozen other teachers went to work at CPS’ Central Office, 42 W. Madison St., where around 1,000 workers picketed daily. CPS’ Passman said those workers are part of a three-year foreign-exchange program and working in Chicago on visas.

Luskin said their immigration status “was being threatened” if they stopped working.