Ditto with Steve. If you can get to the outside of the window, have one person on the inside doing as Steve suggested, and another person on the outside with a "Flat Bar" A.K.A. a "Wrecking Bar". You should be able to get a 6" taping knive under the sash to protect the sill, then fit the flat bar and GENTLY apply pressure while the person on the inside GENTLY bumps the window frame with the meatside of the fist.
GENTLY. I've seen what broken glass can do to a person's hand.....It's amazing how much blood a person can loose in 30 seconds from a severed artery