Dark Moon Gates: Witches' Gates Saga Book 1

Willa, high school Junior and witch-in-training has a problem. No, not the ordinary problem of being a lonely, geekish misfit, though sure, she can add that to her list. And not the problem that she's got a crush on a delicious and evasive lord of Faery, and her mom's got the hots for him too. That's the easy stuff.
Teachers at her school are disappearing. The substitutes are faery Sidhe in disguise, and they're trying to uncover the identity of the child who will, according to prophecy, open the Gates to Faery. That kid happens to be Willa's 3-year old brother Arrie.
Since opening the Gates means the humans will invade and ravage the realm of Faery, the Sidhe plot to sacrifice Arrie at the dark moon, two weeks away. If they manage to do it, the Gates will close forever and the human world will perish for lack of magic. They haven't discovered Arrie's identity yet. They're narrowing down fast.
Then there's that pesky detail in the prophecy: Willa will be struck blind if she manages to stop Arrie's sacrifice.