This father openly admitted that he left the office without signing out his child, after unsuccessfully talking with Mr. Lull, went directly to the classroom to gather his son, which is clearly against the rules. He admitted that
And the minute he did that, mistake or not, he crossed a line that schools should protect. Parents under no circumstance have the right to go directly to the child's classroom. By doing so, he clearly showed he was not acting in a reasonable manner.
Clearly the six year old could have been dealt with in a different manner. I'm sure there's a middle road truth between the parties.
However, adults can't used the excuse of not knowing the rules as the child can. Dad went past his authority when he broke the rule. The school not only has the right but the responsibility to protect the other children in that building and to maintain order.
"in loco parentis" is the legal responsibility of a school to take on some of the functions and responsibilities of the parent, in this case, protecting children from a guest to the building.
I will take the word of WUS that this young man is a good person and simply overreacted and made a huge mistake. People get stopped by the police every day for making mistakes.