AIBU to not want to pick up disruptive Grandson from School.

Well as I read your post I couldn’t help thinking your grandaughter is the blue eyed (as you put it) adorable girl and the boy is the ‘bad’ one
Many boys of 6 are full on with boobs, bottoms and willys I don’t think that’s anything at all to get uptight about but it doesn’t fit in with your well behaved ‘picture quietly drawing’
adorable grandchild belief

If you treat the boy in the way if feels from your description I not surprised he doesn’t behave he feels your irritation and plays up to it he knows he can’t come up to his sisters
reputation so he ll go the exact opposite

The little boy behaves for the other grandparents so I m afraid how you treat him even if it’s not intended is causing the problem If you want him to behave differently you will need to adjust your ways of dealing with him and ditch the favouritism

I'm years past driving children unless an emergency. Forgive me if this is no longer acceptable. I pulled over, stopped, and listened to the radio. I refused to speak or proceed unless my children could remember they had manners and could use quiet voices. An approach all children understand, imo.

Are they allowed a little after-school treat? I usually take something when collecting Gdcs now and then - e.g. crisps or chocolate buttons. They’re always starving anyway.

If so, I don’t know whether it’d work, but I’d say you’ve got treats for them - and show them - but they will only have them if they behave very nicely.
Obviously your Gdd will - if Gds doesn’t, then he doesn’t get whatever it is. No discussion, no getting cross, just ‘Well, I told you - it was up to you.’