I'm trying to identify the title of a book I read in the mid 2000's. It may have been the first in a series. I remember a few specific details and scenes but not much of the overall plot. I read it in English am fairly sure it was originally published in English. I've attempted to track it down through google using various key word searches but have come up empty so far:
- Two teenagers went missing from a school trip when they were transported to an alternate universe (possibly through cairn/stone circle magic - I can't remember clearly)
- They were a boy and a girl. I think the POV switches between them during the book. While they were not friends at the beginning (fairly sure they'd had quite limited interaction previously and even disliked each other to a degree)they become close through the events of the book
- The girls was wearing a baggy hoody, didn't wear makeup and had an angular face I think, when the two were discovered she passed herself off as a boy
- The world they ended up in was medieval in terms of technology, but no stranger to magic. I don't think magic was all over the place though. The two ended up being trained as warriors
- There's a specific scene where the trainees have to walk across a stage or a room as a final initiation or test to become true warriors, and they have to do it naked. The girl manages to partially shapeshift and actually become a man in all the relevant ways so she's not found out (I vaguely recall some surprise from the characters here, she had never done it before and up until that point had only managed some sort of illusions which tricked the sight)
- At the end of the book I think they worked out how to transfer between worlds (possibly involving lightning?) but I think something may have gone wrong and only one of them managed to leave
Any help would be much appreciated!