I don't want to sound patronising but I've always been told (and now tell younger teachers) never to physically stop a child from leaving a room. It leaves you open to all sorts of accusations as well as encouraging them to smack you. They then have a good mitigating circumstance of 'heat of the moment, fight or flight, adrenaline, acting on impulse etc.' which will prevent them being expelled for it.I was reading during my 5 second afternoon break.
Incidentally, one of the delicate little flowers attempted to shove me out the way today as I was trying to get him to stay until the end of the lesson by standing in front of the door (I let the well behaved ones go 5 min early). He was having a melt down and calling me all the names under the sun and at one point I thought he was gonna hit me.
No idea on the environment you're in but I wouldn't want to try that without management I could guarantee would back me every step of the way (and if I find a management team like that I'll let you know).