They're right. School holidays vary from island to island and from district to district even on the same islands. Some schools have year-round school and others don't.

The best times for Hawai'i travel is right now: January (after New Year's) thru about mid February and September (after labor Day) thru most of October.

Avoid June as most mainland schools let out and everyone is off on their first vacations. Not to mention all those pesky weddings on the beach that interupt your sun. Add to the list major holidays like Easter, Thanksgiving, Christmas, New Years and the like.