First, you have no basis for saying that it was beyond his control. You don’t know why he was late. Did he get up late? did he leave his lunch on the counter. Did he do any of the other million things that parents know children do that cause them to be late? You don’t know. You are simply assigning blame to the mother because it supports your emotions.
When a child enters class tardy it disrupts the entire class. If my child is in that class, it diminishes my childs learning experience. His classmates did not suffer for his absence, they suffered for his disruption.
The school has a rule, establishing rules is one of their responsibilities.
The school set a consequence for breaking the rule. Again one of their responsibilities.
The school administered the rule in an equitable fashion.
The child was not harmed, his physical security was not threatened. He bore only the burden of stigma for is breaking the rule. No different than being sent to the office or having to turn a behaviour card from green to yellow or even red.
You roll with that. That kid will be a good little drone, no doubt about it.