How do you solve this math problem?

School A has 20 fewer students than predicted.

School B, in the same district, is over-enrolled by 30 students.

In most systems, the answer would be to leave School A alone and hire a new teacher for School B.

But in the Philadelphia School District, the solution is different. A month into the school year, officials will pull the least-senior teacher from School A and send that teacher to School B.

It's called "leveling" — the process of shifting teachers based on enrollment changes more than a month into the school year. It's virtually nonexistent in other districts but is a thorn in the side of thousands of Philadelphia children, families, and teachers.

After an already rocky start to the school year, leveling is expected to be completed Monday, district officials said. Because the process is still in flux, they could not give specifics, but in the last few years, it has affected about 100 teachers systemwide. (The Philadelphia Federation of Teachers said more than 100 teachers would be affected this year.)

Leveling, said City Councilwoman Helen Gym, is an "outdated practice that needs to end. It has a profound impact on school communities, disrupting the school year, and severing the relationships teachers and students build throughout the first weeks of school."

Uri Monson sees it another way. The district's chief financial officer says leveling is a way to "maximize the resources we can get in schools for kids," an imperfect process that maintains equity among schools.

Monson also said ending leveling would cost the district $12 million that it cannot afford.

When leveling discussions began, Lea Elementary, in West Philadelphia, was told it was losing two teachers, then one. Kitty Heite, a second-grade educator at the K-8 school, was the least-senior teacher on staff and thus was set to be transferred out, she said. (She's now in limbo. Heite was told at midweek that she still could be moved, but not immediately.)

The terms of the teacher contract amplify leveling's effect, school officials point out. In the Lea case, in theory only the under-enrolled sixth-grade classes could be disrupted, but because of seniority rules, both the sixth-grade and second-grade classes will.

Heite and dozens of others in the Lea community — teachers, parents, community members — have organized against leveling, warning against its disruptive effects. The school serves as the overflow building for Penn Alexander and Global Leadership Southwest, two schools with tightly capped enrollment; it is a hub for students with autism and those who are learning English. Its rolls often swell midyear with students from those and other nearby charter schools, teachers say.

"They say leveling is about the numbers, but they overlook the individual experiences of these kids," said Heite, a veteran educator in her first full year with Philadelphia schools. "This is not a practice that happens in healthy school districts. This is not a practice that happens in white, middle-class school districts."

Heite, a long-term substitute at Lea last year, began cultivating relationships with some of her students as soon as she got her class assignment last spring. After weeks of early dismissals and planned school holidays, she finally felt that she was hitting her stride, that the classroom was beginning to gel.

"Pedagogically, this makes no sense," said Heite. And it will have a ripple effect; if Heite is pulled from Lea, sixth-grade classes will be combined, and one of the sixth-grade teachers will shift to second grade.

"It's like removing a piece from the bottom of the foundation," said Tanya Walker, a parent of two children at the school. "Once you move it, everything is going to fall. To have everything constantly changing is just hard."

Kirsten Minor, whose daughter is in Heite's second-grade class, feels lost, she said.

"I worry as a parent: Is leveling going to affect my child academically? It makes me feel like, 'Should I move to the suburbs? Should I move to Jersey?' "

In high schools, leveling can mean large swaths of students, and sometimes a whole school, must get new schedules.

It's a worry for her whole school, said Debbi Marenbach, a teacher at Forrest Elementary in Holmesburg. Prior to leveling, Forrest had to make some early class adjustments to account for large numbers of ESL and special-education students.

"So when we reshuffle to accommodate losing a teacher and collapsing a class, some kids will now be with a third teacher since the start of the school year," Marenbach said. "There is absolutely no acceptable justification for this, and it is certainly not in the best interests of the children."

The majority of U.S. school systems do not use leveling, but some — large urban districts — do. Los Angeles Unified, the nation's second-largest school system, employs a similar practice in the second month of its school year, a spokesperson said.

Some districts have stopped doing it. Chicago, the nation's third-largest district, recently ended the process. If a Chicago public school's enrollment drops in the fall, it cannot lose teachers at midyear; if its enrollment surges, it gets additional resources.

Each Philadelphia school's situation is considered separately, said Monson, with members of the finance and human resources departments looped in, as well as assistant superintendents and principals. Individual situations such as historical enrollment fluctuations and special-education concerns are considered, and in some cases, leveling is taken off the table.

But in the end, ending leveling isn't practical for Philadelphia, Monson said. Besides the equity piece, if the district needed 100 new teachers in October, it likely would be looking at more teacher vacancies, given its existing vacancy list.

"It's our job to look at this systemwide," said Monson. He said he wishes that he could look at 220 individual school situations but that if the system didn't reallocate its teacher resources, "the reality is, it's a $12 million issue."