Did he say the meeting was over and he wanted you to leave or did he just say the meeting was over? In other words, did he ever state to you that you needed to leave the school? If he never actually requested that you leave, you might have a chance of beating it.
The investigation may have simply been asking the principal to describe what happened. After hearing it, the investigator felt that the events did not rise to the level of disorderly conduct so they lowered the charge.
You should be able to obtain a copy of the incident report if they have completed their investigation. You might have file a formal FOIA request, in writing, to get it. In some states, they can simply state the incident is still under investigation (whether it is or not I cannot say) and it is exempt from any requests...even FOIA. If you know a lawyer you could ask them to fire off a formal request for it and see if that works.

