Middle and high school students have to get vaccinated against whooping cough this year. But, as The California Report's Education Reporter Ana Tintocalis tells us, financially strapped school districts are struggling to enforce the new rule.

Ana Tintocalis: Crowds of teenagers eat their lunch in the hallways of Washington High School in San Francisco.

All these students got their whooping cough shot in time to meet the school districts' vaccination deadline last week.

But 19 other students were initially blocked from coming to class. Lynda Boyer-Chu is the school nurse.

Lynda Boyer-Chu: They were called down to the office; they were told there was a clinic that's available today. Or you can get it from your primary care provider.

But you need to bring back with you a pertussis form.

Tintocalis: It's a scene playing out in districts across California, as districts try to force families to comply with the new state law. The measure requires middle and high school students to get the pertussis vaccination -- or whooping cough shot -- within the first 30 days of the school year.

No shot means no school for a student.

In San Francisco Unified, roughly 14 percent of students are still not vaccinated -- or haven't shown proof they've received the shot.

Boyer-Chu believes there are three factors at play.

Boyer-Chu: There is some breakdown in communication between the students and their family. A second is that these families they don't have a connection with health care. And third is that the students and the family are not invested in education for one reason or another. So really the Tdap is just a small issue for these families.

Tintocalis: But those factors don't make a difference under the state law.

San Francisco and other districts that are blocking unvaccinated students are losing thousands even millions of dollars because schools get state funding based on student attendance.

The financial pressure -- combined with the loss of valuable learning time -- is causing some school districts to defy the state law and allow unvaccinated students on campus.

For example, the Natomas Unified School District near Sacramento is allowing unvaccinated students on campus but putting them in the school gym for classes.

Linda Davis-Aldritt: We do understand districts are really hard pressed right now, but the goal is really to keep kids health alive and learning.

Tintocalis: Linda Davis-Aldritt is the nurse consultant with the state Department of Education. She says the vaccination law is meant to prevent another whooping cough outbreak.

Pertussis was a major cause of infant death worldwide about 60 years ago. Childhood vaccinations helped to keep the disease at bay, but public health officials say its coming back -- in part -- because fewer people are getting booster shots.

Dr. John Talarico is with state's Department of Public Health. He says 50 to 80 percent of students in a classroom will contract the disease if they aren't protected. That's why he says there is so much at stake.

Watch a video from the Mayo Clinic that shows a child with whooping cough

Dr. John Talarico: So its not your run-of-the-mill respiratory virus that causes a cold. It is quite severe. And anyone who has it will tell you, you are quite miserable when you have it.

Tintocalis: But enforcing the vaccination requirement is becoming a problem itself -- with the lack of school nurses and support staff on campuses due to budget cuts.

State education officials say they're still reviewing potential penalties for schools that are still admitting unvaccinated students. The state department of public health plans to conduct audits -- but not until December.