I'm looking for a book and i can't remember what it's called?

So this book has a boy and a girl, in high school i think, who don't like each other and somehow get lost together on a field trip walking through this weird mist. They end up in this parallel universe where there's magic. The girl has the ability to shield the boy against magic attacks(i think), and she can change her appearance. She does it to look like a man at first to get excepted by the warriors, and then a girl again, and the boy does this weird berserk thing when he's fighting. They have to get help to fight against this witch and the girl summons the Roman regiment that disappeared off the face of our Earth into their world.