Can I force a middle school to allow my wife and I to view a video which captured the assault of our 13 year old son by another student?

In the absence of a personal injury lawsuit, a discovery request for the video tape as evidence, and a court order to compel production of the video tape if the school refuses, the school is within its rights to withhold the video tape until it is compelled to do so by court order. That's the civil side of the equation.  What about criminal prosecution? Assault and battery is a crime. Did you call the police?  Did you press charges? Did the police conduct an investigation? Did they see the video tape to identify the assailant? Do you know for a fact who the assailant was? How do you know? There's nothing like a badge or better yet a court order to show cause why the school is not letting the police see the video tape when your son's injuries are prima facie evidence of the occurrence of a crime.  If you haven't already done so, talk to the police and be prepared to press charges. Also, if you can do so in a calm, reasonable manner (or better yet retain the services of a competent attorney to do this for you), have a conversation with school officials about the economics of a lawsuit and how far you're prepared to go to seek justice for your son, especially when it could very well be the case that it was the school's failure to properly supervise a student with violent tendencies that resulted in your son's serious injuries. In this modern age in which people are prepared to sue each other at the drop of a hat, it shouldn't come as a surprise that the school is effectively circling the wagons and doing whatever they can to avoid potential liability with respect to the injuries your son sustained.  Be sensitive to that, but remind them that it's just a matter of time before they are compelled to produce the video tape and that it will go better for them if they cooperate now.