The Brockton School Department, known for boosting the academic performance of its poor and heavily immigrant students, says a major budget shortfall will force it to issue 344 pink slips Friday, including 189 to teachers.

School officials warned the cuts will result in larger classes and the elimination of after-school programs and middle-school activities such as band, chorus, drama, athletics, and intramural sports. Technology upgrades will be shelved across the district, bus service will be cut, and no new textbooks purchased, the officials said.

“This is very troubling — the worst I’ve ever seen it,’’ said Aldo Petronio, the school budget chief, who blamed the shortfall on changes in the state school aid formula.

It’s unclear, however, whether the layoff notices will ultimately result in teachers losing their jobs. Brockton officials have a history of sending out large numbers of pink slips in spring and then rehiring most, if not all, of the teachers by the fall.

The layoff notices have the immediate effect of causing widespread panic among teachers, students, and parents, who have mobilized to lobby legislators for more money.

In 2010, 431 Brockton teachers received layoff notices and all but one returned, according to the Brockton Enterprise. Two years later, 100 pink slips went out to teachers and 99 returned. And in 2014, the city issued layoff notices to 199 teachers and all returned, the Enterprise reported.

Mayor Bill Carpenter acknowledged Wednesday that the city has often issued more layoff notices than necessary. He blamed the problem on the teachers’ union contract, which he said requires that layoff notices be issued by May 15, to give teachers time to find other jobs.

By that date, he said, the governor and the Legislature have not yet approved a state budget, so school officials must rely on state funding levels that are not final.

“It’s an artificially early date, so it forces the School Committee to take a worst-case scenario approach,’’ Carpenter said.

In addition, he said, the city has to issue more layoff notices than needed because union rules allow veteran teachers who receive the notices to keep their jobs and bump less senior teachers from their positions.

Still, Carpenter and other city officials insisted Brockton’s schools are facing a $16 million budget gap this year that will force massive layoffs if the state does not send more money.

“There’s no question that some of those years, the majority of the teachers were called back,’’ the mayor said. “What’s different this year is we’re dealing with a $16 million deficit. We’ve never had this big of a deficit. This financial crunch is real.’’

City officials pointed out that major layoffs have been necessary in some recent years. In 2015, for example, the city issued 173 pink slips to teachers and 70 ultimately lost their jobs, according to the Enterprise.

Kimberly A. Gibson, president of the Brockton Education Association, said the city has gone from about 1,435 union teachers four years ago to 1,350 this year, while at the same time student enrollment has increased.

“People are anxious and teachers don’t want to leave Brockton,’’ she said. “They love working here. But every year they seem to do more with less.’’

This year, the city is blaming its shortfall primarily on a change in the state formula used to calculate how many low-income students are enrolled in local districts.

For decades, the state based that figure on the number of students who qualified for free lunches. But in 2015, the federal government prompted the state to switch to a new formula that deemed students “economically disadvantaged’’ if their families receive food stamps or other welfare benefits.

Under the old formula, more than 80 percent of Brockton’s approximately 17,000 students were considered low-income, city officials said. Under the new formula, fewer than 60 percent are considered low-income, a drop that the city says has cost it about $6 million in annual state aid.

City officials said the new formula undercounts immigrant children, especially those who are undocumented and fearful of enrolling in government aid programs.

They also said the opening of a new charter school has cost the district about $5 million in state aid. And contractual pay raises for teachers have added another several million dollars to the budget.

Brockton has been lobbying — along with other communities with large, poor, immigrant populations — for changes in state education aid policy.

“We’ve explained our plight, and the Department of Education is working with us,’’ Petronio said. “It’s not just Brockton — it’s Chelsea, Revere, Lawrence.’'

The crisis is particularly disheartening for a district that has made progress cutting its dropout rate and improving state test scores, he said.

“This is a big shock and we’re still very devastated with all the cuts that we’re getting,’’ said Maria Wilson-Sepulveda, the mother of a sixth-grader in the Brockton Public Schools and a ninth-grader who attends a regional high school in South Easton.

Superintendent Kathleen Smith said, “As we move forward, our goal is to stabilize our district and build it back up to continue to deliver the educational excellence that is rightfully expected from the Brockton Public Schools.’’

Johanna Seltz can be reached at seltzjohanna@gmail.com.