Dwight Harris, 16, an 11th-grader at Ballou High School, pictured outside the D.C. school. (Sarah L. Voisin/The Washington Post)

Nearly 200 teachers have quit their jobs in D.C. Public Schools since the school year began, forcing principals to scramble to cover their classes with substitutes and depriving many students of quality instruction in critical subjects.

The vacancies hit hardest in schools that already face numerous academic challenges, according to data The Washington Post obtained through a Freedom of Information Act request.

At Ballou High School in Southeast Washington, more than a quarter of the faculty quit after starting work in August. Many of their classrooms now have long-term substitutes. Dwight Harris, 16, an 11th-grader, said his Algebra 2 class has been chaotic since his first teacher left in January.

“No one is teaching. It’s been like that for months now,” Harris said. “We don’t do anything, so I leave and go to my biology class or English class and go do other work.”

Most teachers wait until summer to call it quits, but in DCPS a rising number are leaving during the school year.

The mid-year resignation rate for DCPS was higher than for some other urban school systems The Post checked. In the D.C. system, 184 of about 4,000 teachers — nearly 5 percent — quit from September to mid-May. That was a 44 percent increase over the 128 teachers who left in the 2013-2014 school year.

In Denver Public Schools, which employs about 4,600 teachers, 115 teachers left in a comparable period this year. In Baltimore City Public Schools, with about 5,150 teachers, the total who quit was 145. In Seattle Public Schools, with about 4,000 teachers, 55 quit.

DCPS spokeswoman Michelle Lerner acknowledged it is a challenge to lose teachers mid-year. School officials try to fill vacancies as quickly as possible with a full-time teacher, but she said the best time to hire is in the summer.

“Having a high-quality teacher in front of every classroom is a huge priority for us,” she said.

While the number who quit abruptly is small compared with the total workforce, experts say mid-year resignations are particularly disruptive and harmful to student learning because it’s very difficult to fill sudden vacancies.

Most good teachers are employed during the school year. That means if a teacher leaves mid-year, classrooms are left to a rotation of short-term substitutes or a long-term sub who may not be fully qualified to teach at that grade level or in a specific discipline, such as math or biology.

“Every teacher, no matter how successful they are at their job, knows that leaving mid-year is a really unkind thing to do to kids and the school. If they are doing it, it’s out of anger, or an overwhelming sense that you are not doing anybody any good by staying,” said Kate Walsh, president of the National Council on Teacher Quality.

The Post obtained two sets of data on DCPS teacher resignations. One covered the systemwide totals for the past four school years. The other, from the FOIA request, showed in detail how many teachers quit at each campus this school year from August through February. Students started classes on Aug. 22, but teachers reported to work earlier that month. Hiring typically occurs by the end of June.

In most DCPS schools, the faculty is stable. Of 115 schools in the system, 59 had two or fewer resignations after teachers reported to work, the data showed.

But a handful were hit hard.

Columbia Heights Education Campus in Northwest lost 11 teachers, or 10 percent of its faculty. H.D. Woodson High in Northeast lost 10 of its 50 teachers, or 20 percent.

No school has suffered more turnover than Ballou High. It lost 21 teachers from August through February — 28 percent of its faculty. Many of the resignations occurred in the math department, current and former teachers say.

Several former Ballou teachers told The Post they did not want to leave mid-year and felt bad about the consequences for students. But they said a number of problems drove them to leave, from student behavior and attendance issues to their own perception of a lack of support from the administration. They also raised questions about evaluations. Some veterans said that in previous years they had received high marks from administrators, but this year they were given what they believe are arbitrarily low evaluation scores.

DCPS officials declined to make the principal of the school, Yetunde Reeves, available for an interview.

Lerner, the spokeswoman, said the school system is looking closely at the Ballou situation.

“We are working with the school to make sure that the staff in the building feel supported and to create a long-term vision so we don’t continue to see high turnover at Ballou and other schools,” she said.

Rowan Langford was the Algebra 2 teacher for Harris when the school year began. The 22-year-old was a teaching fellow at Ballou. It was her first teaching job after graduating from Tulane University with a bachelor’s degree in math.

Former Ballou High math teacher Rowan Langford. (Evelyn Hockstein/For The Washington Post)

Langford said she asked administrators for help with behavior problems in her classroom — but didn’t get it.

Her classes were large. One had more than 33 students. She said the students were very far behind and lacked the foundation needed to be successful.

“A lot of them felt really discouraged about math and used other methods to lash out,” Langford said. “I couldn’t address those problems they were having on my own.”

Langford said she threatened to quit two months into the school year but was hopeful she would get support to manage her classroom. She said nothing changed. In January, she decided to quit.

“I felt awful about it,” she said. “Before I started this job, I said I didn’t understand why anyone would quit mid-year. But being in it, you realize how long a year is because every single day feels like three.”

Ballou has about 930 students, and all qualify for free or reduced-price lunch because they live in poverty. Many come from homes where their parents didn’t go to college. The school ranks among the city’s lowest-performing high schools on core measures. Its graduation rate in the last school year, 57 percent, was second-lowest among regular high schools in the DCPS system.

In 2016, 3 percent of Ballou students tested met reading standards on citywide exams. Almost none met math standards.

The school was reconstituted in the 2015-2016 school year, its second shakeup in five years. Reconstitution means the teachers and staff all had to reapply for their jobs.

Yetunde Reeves, Ballou principal (Bill O'Leary/The Washington Post)

Principal since 2014, Reeves recently said she and her staff were working to change Ballou’s image by raising expectations for students. In March, the school said all of its seniors had applied to college, a first for Ballou.

Monica Brokenborough, a music teacher and the school’s union representative, sent a letter this month to the D.C. Council, Mayor Muriel E. Bowser (D) and DCPS Chancellor Antwan Wilson raising concerns about the staff vacancies.

“Students simply roam the halls because they know that there is no one present in their assigned classroom to provide them with an education,” Brokenborough said. “Many of them have simply lost hope.”

Richard Ingersoll, a professor of education and sociology at the University of Pennsylvania and expert on the nation’s teacher workforce, said there is no national data on what portion of teachers leave in the middle of the school year. But he said a quit rate as high as Ballou’s signals “there are some problems in that building.”

Ingersoll’s research shows that teachers who resign abruptly often do so because they do not feel supported by their administration. Some may leave if they do not feel safe in schools where there are fights and other disruptions. Those issues take a greater toll on inexperienced teachers.

“High turnover, whenever it happens, suggests there are problems in the workplace,” Ingersoll said. “If it’s in the middle of the year, that suggests things are so bad people can’t wait until the end of the year.”

Walsh, with the National Council on Teacher Quality, agreed that “leadership is everything.” Walsh said that when a large group of teachers leaves mid-year, many could be “disgruntled” and that students may be better off if ineffective teachers leave. Still, she said the school system needs to examine what is driving teachers out.

“I imagine behind closed doors, they are questioning leadership,” Walsh said. “They ought to be using it as a point of discussion with those school leaders.”

Harris said that since his teacher left, he hasn’t learned much in algebra. Substitutes have told him and his classmate to fill out worksheets, he said, which they answer by Googling the problems.

Many times, Harris said, he stays in the room for 10 minutes, long enough for the sub to mark him present.

“I have no idea what my grade is right now,” he said, “but I think I’ll pass the class.”

Asked about Harris’s class, Lerner said that students in it are still receiving instruction. The school is “on a watch for how students do,” she said, and if there is any loss of learning officials will add extra time to the next year’s schedule for math instruction.

In her message to city officials, Brokenborough included handwritten letters from students who described feeling unprepared for their Advanced Placement exams and fearful that their prospects for college will be hampered by not having a teacher in key classes.

Iyonna Jones, an 18-year-old senior, said in one of the letters that security guards tell the students lingering in hallways to go to class, but she has a substitute teacher in her math class and doesn’t feel she is getting the instruction she needs.

“We should just stay home, because what is the point of coming to school if we are not learning and have no teachers,” she wrote.

A previous version of this story cited statistics on teacher resignations at Raymond Education Campus. D.C. school officials subsequently clarified that the school is on a year-round schedule — unlike most others in the system — and that the turnover there was not unusual. This version has been updated.