LOS ANGELES — Standing before the Los Angeles Unified School Board, Susan Zoller delivered a startling assessment: More than 100,000 students in the nation’s second-largest district are now enrolled in charter schools, draining more than $500 million from the budget in a single academic year.
‘‘The financial future of Los Angeles is difficult,’’ said Zoller, a consultant hired by the district’s union. Board member Richard Vladovic leaned anxiously toward his microphone.
‘‘We are bleeding,’’ he said.
If current trends continue, the district could be significantly diminished in another 10 years — at least a third smaller than at the start of the century.
In financially struggling urban districts from Los Angeles to Philadelphia — and most notoriously, Detroit — the increasing popularity of charter schools, combined with family flight to the suburbs and declining birth rates, have caused enrollment to plummet. The changes have unfolded slowly for years and recently accelerated in some places.
‘‘It’s come to a tipping point for many of these districts like Detroit,’’ said Ron Zimmer, an education professor at Vanderbilt University. ‘‘They just can’t finance their school district that was meant for a much bigger enrollment than they currently have.’’
Charter schools arrived in the 1990s and began attracting parents searching for an alternative to big-city districts that had strained for years to raise performance among minority and low-income students and those who are learning English.
More than two decades later, charter enrollment continues to climb. Nationwide, more than 2.6 million students attended charter schools in 2014, according to the National Alliance for Public Charter Schools.
In districts with growing student populations, such as Las Vegas and Orlando, that growth helps ease potential overcrowding.
But in cities like Los Angeles, where the school-age population has been shrinking, the continued flight from traditional public schools has become a mounting concern. In most states, schools receive funding on a per-pupil basis, and the majority of those dollars follow students when they leave for a charter.
Charter school advocates say it’s only fair for local and state property tax dollars to follow children to the new schools, and that parents aren’t to blame for a district’s failing finances.
‘‘To the extent the district is not serving the needs of their students, this has been a trend line for some time,’’ said Nina Rees, president of the National Alliance for Public Charter Schools, a nonprofit advocacy group.
District leaders contend that even with fewer students to educate, they still have a range of fixed costs. Ultimately, they say, the funding decline affects programing for students still at traditional public schools, who often face the steepest challenges.
If Los Angeles schools are no longer able to function as a district, ‘‘there is going to be collateral damage,’’ said Steve Zimmer, president of L.A. Unified’s school board. And the damage ‘‘will be to those children and families who are the most vulnerable.’’