I think it is a YA novel, I remember reading it in middle school. I'm a bit fuzzy on the plot, but I remember a Princess jumping into a lake and time traveling to modern times. She goes to a high school and compares modern games and items to medieval ones and develops a romance with a boy. I'd like to find the book again, but I cannot remember the title, author, or characters names. Any help would be most welcome!
Princess Nevermore by Dian Curtis Regan, maybe.
"The worlds of the fairy-tale past and the workaday present collide in this suspenseful and poignant fantasy about a magic princess who briefly sojourns in middle America. Sixteen-year-old Princess Quinn's only diversion in the underground kingdom of Mandria consists of watching the "outer-earth" people through the waters of a wishing pool. Betraying his master's trust, an apprentice wizard sends Quinn to the outer world, where she meets up with teenagers Sally and Adam Dover and their mysteriously wise grandfather Mondo."
You can see the book covers here: http://www.librarything.com/work/87363/covers