Many parents said they hadn't heard of the immunization requirement until this week.

Some California schools are turning away middle and high school students who have not received a required whooping cough vaccine while others are defying a law passed last year after a historic spike in cases of the potentially fatal disease.

The law approved last September 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 this summer as districts worried many students wouldn’t meet the deadline.

Under California law, students also can still attend if their parents file a form saying they oppose vaccines.

No statewide estimates of the number of students turned away is available because districts are not required to report their final vaccination tally until December, state education and public health officials said.

But anecdotal reports from individual districts indicate the percentage of students meeting the requirement varied widely, from about half of students to nearly all.

“We’ve done a tremendous amount of outreach with the schools trying to let them know,” said Linda Davis-Alldritt, the school nurse consultant for the education department.

On Thursday, San Francisco Unified School District began sending home students who arrived without proof of vaccination or a parental personal belief exemption.

District spokeswoman Heidi Anderson said the district estimates about 2,000 students, or 10 percent of the student body, are still unvaccinated. The district held a free vaccination clinic at its offices Thursday and was providing shots at individual schools Friday.