This is the third time, in a year and a half, you’ve had to ask him. He has the gall to act like you’re being irresponsible??? He left his 8 year old sister alone, when it may well have been getting dark and you, presumably, wouldn’t have been able to get there time. He left an 8 year old on her own, to leave the premises on her own, and go about a location she may not be too familiar with.
I’m the same age as your son, if I had a little sister or brother her age, I wouldn’t necessarily drive (as I don’t have a license yet) but I would march my way down to that school and walk with her back home. I get 6 months in a relationship can be exciting, but ffs, he couldn’t be arsed to lift a finger and tell his partner “hey my dad won’t be able to pick up my little sister from school, so I have to. I won’t be long, you can wait here or come with me” or even just, yknow, walk???
Maybe it’s teenager hormones but reading this made me fume. An unexpected incident doesn’t make you a bad parent, this isn’t your fault OP, you had no way of predicting something would happen, your kid’s just being cruel.
That being said, it might be a good idea to find someone (other than the woman who picked your daughter up in the end because, uh, asking your 8 year old maths questions and then reprimanding her for getting an answer wrong is something we call deeply damaging) who you trust would be able to pick up your daughter in lieu of yourself. Still, I want to reiterate; you’re not a bad parent OP, your son’s just being unjustly cruel.