I find you approach to this bizarre.
The school have a policy that they don't let a child your age walk home alone. Amendments for you, means amendments for others so I can see why they don't want exceptions. Thry have safeguarding duty, it nay even be linked to their insurance. Whether their policy is right or wrong, it debatable.
However, there are multiple simple solution to this.
If DH is WFM, I don't believe that he is so rigorously monitored that a 4 minute break would be a big issue? (Surely he goes to the toilet during the day!) and even if it were, I don't believe that any organisation wouldn't allow him make up those 4 minutes before his morning start, after his normal finish or by reducing his lunch break.
Instead you have opted for nuclear option. Involving the police? No wonder they told you to go away.
You say it's not about being right? It it weren't, your husband would find the 4 minutes and you guys would respond normally.
Your poor daughter in all this. She's probably distressed that no one will bother collect her and would prefer to fight with the school.
I highly suspect that if anything did happen to her, you would be the first to want to sue the school.