OAKLAND  (AP) — A growing teacher shortage has left district officials across the San Francisco Bay Area scrambling to fill classrooms, with critical shortages in science, math, special education and bilingual education.

Many parents will find a substitute teacher, or even a credentialed central office staffer, leading their children’s classes when students begin returning to school this week, the San Francisco Chronicle reported Sunday .

“This is probably developing into a historic shortage nationwide,” said Troy Flint, a spokesman for Oakland Unified School District. “We’re operating in a more competitive pool just to retain our employees and hire new employees.”

As of Friday, just 10 days before the start of school on Aug. 24, Oakland needed 77 teachers, and — barring a miracle — will have many unfilled positions when students arrive. Administrators have launched a military-like response to address the shortage, one that includes “deploying” long-term substitutes and central office staff and “overloading” classes in grades four through 12.

Retirements, high attrition rates and a lack of new recruits are all contributing to the shortage. In 2008, just as the recession started, almost 45,000 people were enrolled in teacher preparation programs in California. By 2013, there were fewer than 20,000, according to the state Commission on Teacher Credentialing.

Oakland, San Francisco and other urban districts started feeling the teacher shortage last year, with high-flying housing costs driving teachers further afield.

Making things worse, the sizzling post-recession economy boosted school budgets, allowing districts to add positions and reduce class sizes again — creating even more jobs to fill.

In San Francisco, the district had to fill 465 teacher positions for the coming school year, compared with 400 last year. There were more retirements and resignations this year, and more teachers are taking sabbaticals, according to district data.

With school starting Monday, the district had 22 positions open last week, and filled the last one Friday night.

Labor leaders said they worry there will be major teacher shortages in coming years, fueled by untenable housing prices in San Francisco.

With teaching jobs available in virtually every district up and down the Bay Area, San Francisco teachers have been leaving for higher salaries and cheaper rent.