It's a reply of children because adults want to be angry with them, not find out why they did something, so people like you tell them to "just be responsible" etc without ever listening or offering support.

So if an adult wants to yell at you, you say "i dont know" and let the angry windbag rant and rave. From the kid's POV the adult/teacher don't care about the reason (y'all usually don't) you just want to have your ego bruised by a whole ass child dealing with whole ass life not about you being late.

But "i dont know" is easier than "mom has a new boyfriend who keeps me up all night and i sleep late" you still probably wouldn't listen or would then tell the child how they should handle the neglect of their education by their parents themselves, rather than show empathy and understanding.

Because it's about them listening to you, not about them being tiny people with no emotional regulation or rights trying to navigate a world where they are treated like little adults with zero power to escape abusive situations in school or at home of their own power, unlike you an adult who isnt the property of your parents.

But yeah they're just stupid little twerps looking to pull one over on you, right? They dont have complex lives outside of their time with you, they spend all day making sure how to make you feel disrespected by showing up late!

And that is why you get "i dont know".