DANBURY - The surprise addition of 350 kids in public schools so far this year is catching leaders off guard in a city that is used to planning for unexpected enrollment increases.

Schools Superintendent Sal Pascarella, who was planning for an enrollment increase of up to 2 percent this year, has seen demographers’ projections badly beaten by a 5.6 percent increase in students.

“In the last 30 days, we have witnessed an unprecedented and alarming increase in new student registrations that will further strain our ability to provide an appropriate education to our students under our approved budget,” Pascarella wrote to the city’s Board of Education.

Pascarella said he plans ask state Education Commissioner Miguel Cardona for a larger share of state aid, for what Pascarella calls “a highly unusual and wholly unexpected situation.”

Until an answer comes from Hartford, Danbury City Hall will have to step in.

Mayor Mark Boughton said it will take about $600,000 to hire new teachers and cover the cost of accommodating the unexpected students, many of whom are enrolled at Danbury High School - already the largest in the state.

“When you divide up this enrollment increase over 13 schools, you wind up with a couple of kids more in a class and it doesn’t necessarily mean you have to hire new teachers,” Boughton said. “But there is definitely a large increase at the high school, so once we have our audit done I will see if there is any more surplus we have to put towards that.”

Boughton, a Republican who is running for an unprecedented 10th term in November, said the surge of unexpected students will be factored into long-range planning already underway.

For example, city leaders are already talking about going to voters in 2020 for money to build new elementary classrooms.

Democratic mayoral challenger Chris Setaro has made the schools’ swelling population one of his major campaign themes. Setaro faults Boughton for not regulating residential development growth on the west side.

Setaro in a statement on Wednesday also criticized Boughton for limiting school funding earlier this year to keep taxes down.

“Because of an election year budget, our superintendent of schools has been left begging for help so our kids can get the education they deserve,” said Setaro, who estimated the enrollment surge would cost $1.3 million to cover.

“I don’t see how it is possible that this was something no one anticipated,” Setaro’s said. “The city approved thousands of condominium units over 18 years of the Boughton administration. How could the planning be this misguided?”