One would assume they opted out because they had concerns about being vaccinated.  https://www.greatergoodmovie.org/

Many people have concerns about vaccines and the potential dangers.  I've never had any problems as a result of mine, but I don't get a flu shot every year like many people do.  I had the standard vaccinations as a child, but there weren't as many of them back then.

The FDA says vaccines are safe.  The FDA says GMOs are safe.(Bt corn has the potential to turn our guts into pesticide factories.)  The FDA says aspartame is safe.(It makes your brain cells explode.)  The FDA says MSG is safe.(I know from personal experience that it's not.)  The FDA says black salve is snake oil.(I know dozens of people who have been cured of cancer by it.)  In short, I don't trust the government or the medical establishment to know their butts from their funny bones.  I'm just sayin'.

@nmlgr- I agree with you about flu shots. I haven't ever had one and, given that the efficacy of them changes from year to year, not 100 percent convinced that they are required to have. Also with you on the FDA, which is another really interesting discussion- have at it if you want to write a top level post about the FDA, GMOs, etc. 

But I do believe that it should be a requirement that certain standard vaccines be taken before a child can attend public school. Polio, measles/mumps/rubella, etc. I saw a PBS special on polio a year or so ago and had no idea of how many people, including children, were crippled or died because of this. 

In the summer of 1950 fear gripped the residents of Wytheville, Virginia. Movie theaters shut down, baseball games were cancelled and panicky parents kept their children indoors — anything to keep them safe from an invisible invader. Outsiders sped through town with their windows rolled up and bandanas covering their faces. The ones who couldn’t escape the perpetrator were left paralyzed, and some died in the wake of the devastating and contagious virus. Polio had struck in Wytheville. The town was in the midst of a full-blown epidemic. That year alone, more than 33,000 Americans fell victim — half of them under the age of ten.

And now, polio is virtually eradicated, so is smallpox, And getting measles, as r u serious says, used to be really serious. From 1861

Measles sickened troops on both sides of the American Civil War.

“During the first year of war there were 21,676 reported cases of measles and 551 deaths in the Union Army alone. Deaths were primarily from respiratory and cerebral (brain) involvement. It was recorded, ‘This infection is always serious, often fatal either directly or through its sequelae. The Prognosis therefore should be guarded.. .“The American Civil War was the last large-scale military conflict fought before the germ theory of disease was developed… Two-thirds of soldiers who died in that war, 660,000 in all, were killed by uncontrolled infectious diseases. Of these, in the Union Army over 67,000 had measles and more than 4,000 died.”

–Michael B. A Oldstone, Viruses, Plagues, & History, 146-47 (2009)

And the study that purported to link autism to the MMR shot has been debunked.

To me, the duty of a public school, that I pay for with my taxes, is to provide a safe, healthy environment for children who are compelled to attend.  And maybe it's saying that I want there to be a standard of given mandatory shots that we ALL agree upon are necessary.