OK.
You cannot catch chicken pox twice. It mutates at a very slow rate and once you've got the antibodies that is it. Every case of folks thinking they have had chicken pox twice are folks who have misdiagnosed similar illnesses or allergies.
It can be nastier when you get it as an adult but the myth of "if you don't get chicken pox as a child you'll catch it as shingles as an adult" is just that, a myth. In fact the converse is true: you cannot get shingles as an adult unless you have had chicken pox as a child.
Chicken pox is a herpes virus like coldsores and herpes. Once you've been infected it withdraws to nervous system. It can then return infecting you along the line of the nerve; shingles.
There is a chicken pox vaccine but I don't know if it is a live or dead vaccine off the top of my head. If it has to be a live one then I'm guessing it carries the risk of shingles whereas a dead one wouldn't but that is just speculation from me. I know that there is also a shingles vaccine.
Given how serious it can be for those with suppressed immune systems, pregnant women, the old and the very young I am eternally shocked by the concept of chicken pox parties. I am particularly pissed off with the lie that "I'm doing it to stop them from getting shingles when they are older" when they are actually increasing their child's chance of getting shingles from 0 to 100% (not 100% chance they'll get it but 100% chance they could get it).
Ignorance about diseases is a real issue and as usual the hate mail and other newspapers do not help. I think this is something that needs teaching more in schools. I've already written in a couple of lessons to look at lies in the media about MMR etc and bring up Dr Mr Andrew Wakefield and Jenny McCarthy and how lies and poor science are over reported whereas real science not always being as sexy can be ignored. During that I do touch on how serious some childhood illnesses are and go into detail about fatalities due to Measles and Mumps and, of course, the ticking time bomb of having a big chunk of society not having immunity to rubella. The lack of herd immunity to rubella among an ageing population of girls is a disaster waiting to happen.
I will write up some work on the truth about a range of "childhood" illnesses but it will only be one school.
So YANBU.