Whilst I don't have any jurisdiction over the behaviour of parents on the street, I do have responsibility for the pupils' behaviour on their way to and from school.
"In the spirit of this I would respectfully ask that adults refrain from smoking immediately outside the school as some may see this as setting a bad example."
Do you really think this is apportioning blame on others or deserves abuse?
Stop being offended on others behalf, there is no abuse in that post.
I still stand by the duty of care argument. A teacher cannot legally be held responsible for what happens outside of the school gates.