I deserved every smack I got (and didn't get), but my two cents (or more):
1. Make sure you have all YOUR bases covered (son's not lying, no prior incidents/issues with teacher, etc) so nothing surprising can pop up
2. Lay out the claim for them (them being the school board or whatever) with all your evidence:
- Son was afraid of teacher and therefore acted appropriately by waiting and addressing it with another teacher (student trusts teacher which is good!)
- Witnessed by at least one other student
- Parents were not contacted when student informed them of abuse (and you better believe they would have called you if YOUR son hit the TEACHER)
- Such an emotional teacher (since that was the excuse for not handling in a timely manner) maybe should not be in charge of 26 students (even with aids)
- No precautionary action taken by the district (often a teacher (or student) would be suspended with or without pay until the accusations were resolved unless it could be immediately determined that the accusations were false)
3. Bring all the emails and a timeline of events to any meeting
4. Let them decide the appropriate action, but make your expectations clear (true apology to child, parent, etc. in person not email)
Those are my amateur "legal" opinions
My personal opinions:
- A smack is small (in the large scheme of things that COULD happen these days), but teacher's are expected to remain calm even under pressure (think fire drills, shooter in school, etc) and a noisy classroom should NOT be a pressure situation for a teacher
- An email is a cop-out. Accusations such as this should be handled face to face and in a timely manner. Also, the email lays out the exact details of that day but the only thing she DOESNT remember is slapping your son....interesting....*ponders*
- Be proud of your son. He definitely did the right thing by approaching another teacher with the issue and I can imagine he did NOT want to confront the accused teacher for fear or getting smacked again (at least that's how I probably would have felt). I would also compliment the dean for taking it seriously and the teacher who he confided in for not covering up for another teacher and telling your son to drop it.
I worked at a daycare and was in charge of 23 three-year olds with the help of 1 other person. One thing it taught me is patience and letting things roll off you. We had parents whose children made accusations and luckily the parents were responsible, approached us or the director, and usually discovered their kids may not have told the whole story. I never got defensive because I knew I had never done anything wrong and keeping a level head usually shows the parents your ability to control situations that arise. I watch 3 special needs kids for a couple hours each week (one of which has hearing problems, anger management, and ADHD issues). He'll throw ridiculous tantrums that many parents would not be able to handle, but I've never even raised my voice with him. Calmness always prevails. The only thing that can make my blood boil is a roughing the passer/kicker call in football these days!

(My fiance actually gets upset with me because she thinks I remain too calm at times and should get angry more LOL)
Good luck!