My understanding of the chickenpox vax is that breakthrough pox is fairly common - however it usually a much milder form of the disease - fewer spots, lower temps, get better more quickly and are less likely to infect other children (because the total germ load is less) although still contagious.

FWIW there's no way to know who your child "caught it" from. Chicken pox is fabulously contagious (via sneezing & coughing) even before the spots appear, so there are likely to be a number of children at the school who have it.