Phil positions both of the roommates to make it look like one killed the other, and then committed suicide. "There we go, now to call 911!"
A little bit later the police arrived and inspected the area, running scans on fingerprints, footprints, etc.
One officer used the knife on the fingerprint scan and found Phils prints on it! "It was the student! Put your hands in the air!"
Phil surrenders and goes to prison for 20 years with no parole.
THE END!