Ah here is the clause!
the only way a child can get exemption is very strict:
http://www.immunization-sd.org/docs/iz58.pdf
There are 4 ways:
permanent medical Exemption - must have medical condition
Temp Medical Exemption - must have medical condition
Measles, mumps, rubella exemption - must already have been infected
Chickenpox Exemption - must have already had it
So the only way to really get exemption is to already be infected or have medical condition LOL
so yeah, get exempt if you already have the damn disease!