Mainly by Moonlight

Murder and Magic Make Devastating Partners
Molly Adair is a warrior mage. Her parents are dead and she's just moved to Portland, Oregon to live with her grandmother. She is so totally not looking forward to starting her junior year at Grant High School. She doesn't know anyone and even if she did, she would have nothing in common with them. Things improve immensely when she discovers that Grant is the Mage Web's magnet school for the United States' West Coast. She begins making new friends, both mage and normal, and attending Tesseract Academy, the Mage Web's school at the center of the circle of time.
But then, someone steals Flick, her sentient sword, from its pocket between the worlds and stabs her best friend through the heart.
And Molly is everyone's prime suspect.
Only a powerful magic user could have stolen Flick. And because adept mages can jump through both time and space, alibis mean nothing.
Molly's search for her friend's killer reveals a dark and dangerous secret, forcing her to either find the murderer or become his next victim.