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CHICAGO — Because of a massive drop in enrollment, Chicago Public Schools officials Monday laid off 140 teachers and 109 members of schools' support staffs, district officials said.

The teachers union says that the layoffs make a strike, perhaps as early as Oct. 11, more likely.

The job losses were prompted by a 3.5 percent drop in CPS enrollment between the 2015-16 school year and the 2016-17 school year.

During the last 10 years, enrollment has fallen 6.8 percent at public schools in Chicago, with half of that drop coming since September 2015, according to district enrollment data.

The drop in enrollment was "steeper than expected," district officials said in a statement Monday.

CPS enrollment dropped from 392,285 during the 2015-16 school year to 378,481 this year for a total drop of 13,804 students, district officials said.

In July, officials said they expected the district's enrollment to drop by just 5,240 students.

Laid off teachers and staff members can apply for positions that are vacant. There are 362 vacant teaching positions and 266 vacant school support staff positions, officials said.

Union spokeswoman Stephanie Gadlin said the cuts are "unacceptable and no way to run a world-class school district."

"Today's cuts are the latest round of attacks on children and are pushing educators closer to Chicago's third school strike in four years," Gadlin said, referring to the seven-day strike in 2012 and the one day work stoppage in April.

The enrollment drop means cuts at 306 schools throughout the city will total $44.9 million, according to a statement from district officials. However, schools where those cuts "would prevent them from offering critical academic programming" will get an additional $12.5 million, district officials said.

District officials said Sept. 26 that additional help for schools would total $5.7 million and 300 teachers and support staff would be laid off because of the drop in enrollment. School principals can appeal budget decisions made by CPS administrators.

Because the district funds schools based on the number of students they have, 195 schools will gain $20 million, district officials said.

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In all, Chicago schools have seen their budgets slashed by approximately $170 million since September 2015, according to data released by the district.

In August, CPS officials laid off nearly 1,000 teachers and members of the district's support staff.

Chicago Teachers Union members are set to walk off the job Oct. 11 unless a labor agreement is reached.

The $5.4 billion Chicago Public Schools budget, adopted in August, relies on a contract proposal rejected by the union in February that would raise teachers' pay but require them to contribute more to their pensions.

A school-by-school breakdown of the lay offs is expected to be released by CPS officials late Monday.