California schools are turning away middle and high school students who have not received the whooping cough vaccine.
Students are now required to get the vaccine under a law passed last year after a historic spike in cases of the potentially fatal disease.
The law initially required all students entering grades seven through 12 to get vaccinated by the start of the 2011-2012 school year. Lawmakers passed a 30-day extension as districts worried many students wouldn't meet the deadline.
Students can still attend if their parents file a form saying they oppose vaccines.
San Francisco Unified School District on Thursday began sending home students arrived without proof of vaccination.
District spokeswoman Heidi Anderson says the district estimates about 2,000 students, or 10 percent of the student body, are still unvaccinated.
