The parents can opt out. If the "opt out" procedure is a big hassle and involves a visit from children's services then you have a point, but that doesn't seem to be the case here.
Why make it law that this vaccination is required? This should be a decision made by parents with the advice of their childs' physician. I do not need an elected official, with ties to the manufacturer of the drug, telling me that my children must have it, even if I have the ability to opt out. If the shot is truly optional, then there is no need for the legal 'requirement.' The drug companies know that most parents will get a letter from the school saying that there child needs to get a vaccination and they will comply with no questions asked.