You need to consult with a lawyer because this seems like a very unlikely situation to have a school principal go seeking a truant child. The principal had no right to enter the house in the first place unless he was invited in - then he is like any other invited guest.
The failure to read a juvenile (or anyone) their rights results only in the suppression of any incuplatory statements made to police upon questioning by police while in custody. The case can still be prosecuted. It is probably better that your son did not make a statement because now he can think clearly and be able to articulate what happened to him in a calm way rather than after what he had been put through (which might have caused him to fail to mention some facts, etc.)
Your child needs a good lawyer to represent him. If the case is won on the charge of assault, then you may consider consulting a lawyer about filing a case against the principal, and possibly against the school. Sounds like this principal is out of control, and it does sound like abuse - in fact, if your child's version is correct, it sounds like the principal assaulted your child.
If the principal did put his hands on your child at your house, then your child has the right to defend himself against the principal's unlawful assault. My guess, however, is that this is going to be a completely different version than the one the principal will offer (and that in the principal's version your son caused the breakage of the window and his own injury.)
Sounds like an out of control principal. You definitely need to consult with a civil rights lawyer and complain to the school board. You may have a suit for assault and false imprisonment.
This is intended as only general legal advice. Feel to visit my website, www.austincriminaldefenseattorney.com for my blog on crime and punishment in Texas You may also email me through my site.