Measles has a "90% secondary infection rate in susceptible domestic contacts". So, its likely that his brothers were infected as well. That leaves five others.

Measles has a "90% secondary infection rate in susceptible domestic contacts". So, its likely that his brothers were infected as well. That leaves five others. With the mentioned "97% vaccination success rate under ideal conditions", the fact that they were mixing around in school (and those they infected as well), it does not seem a stretch.