Well, even under the best conditions, the vaccine is only effective 95-97% of the time. That means that in a school of 300 kids, up to 15 would be unprotected, even though they were vaccinated. Since measles is EXTREMELY virulent, it is very likely that these kids would develop measles if they shared a classroom with a contagious child.
While you may have heard of one outbreak where none of the non-vaccinated kids got it, that doesn't seem to be the norm:
"There's definitely a lack of appreciation of measles and what it can do," said Dr. Paul Offit, director of the Vaccine Education Center and chief of infectious diseases at Children's Hospital of Philadelphia. He said that in a recent Indiana outbreak, a 17-year-old girl was infected with measles on a trip to Romania, and when she came home she went to a church picnic attended by about 500 people. Of that group, 35 were unvaccinated. Three people out of 465 vaccinated got measles, while 31 out of the 35 who were unvaccinated contracted the disease, Offit said.
"That's how highly contagious measles is," he said, pointing out that these people were at an outdoor church picnic, likely without sustained exposure to the infected teenager.
http://health.usnews.com/health-new...outbreak-triggered-by-unvaccinated-child.html