I prefer positive persuasion as opposed to restrictions. After all, the parent of an immune-suppressed child will not logically depend on thinking other children are all immunized to protect her child. That would be unwise.
The best way to handle the immunization thing is to educate parents, but not to restrict their movements (in my opinion).
It's a slippery slope. When parents cannot enroll non-immunized students, how close are we to banning non-vaccinated people from grocery stores or other public places? When flu season comes around, I get a flu shot, but I know many adults who do not. Yet, the flu can kill as many as 45,000 people in a year. Much more dangerous than a few kids not being immunized when we have herd immunity in society.
If I was a parent of a child with weakened immunity, the last thing I'd do would be to take her to public school, the grocery store, the movies, etc., if there was a risk of any sort of outbreak. After all, a common cold could take her down, and there's no vaccine for that. What I wouldn't do is insist that non-immunized children not be allowed to attend because I want my child to be safe. That would be wrong to my mind.