In the Nick of Time - Robert Swindells

Charlotte is out in the woods on her own one day when something mysterious happens - she walks along a row of stones laid like stepping stones on the forest floor . . . and finds herself in another age. She has somehow slipped back to 1955, and is now, in the same woods, on the site of a very unique school, an open-air school for sick city children. No one believes her tales of the world she''s come from, her mobile doesn''t work and she can''t see how on earth she''s going to get back. A friendship with another pupil proves the key - is Jack more than he seems?