Yelling "duck" at the shooter as a command may help to bend the knees and lean forward.
Also, most shooters doing the leaning back do it for two reasons: (1) the shooter is scared of the bang and/or (2) the shooter has a lack of upper body strength. If the shooter is scared of the bang, tell the student that leaning back doesn't get him/her any farther away from the firearm. If the shooter has a lack of upper body strength, tell the student to utilize the low-ready position more often to rest (especially with shotguns and rifles).
In general, I am for not teaching any specific "shooting" stance. Instead, the basics of self defense without a firearm apply. If I, as an attacker, can knock you over with a simple push with my hand, so can any firearm recoil.