To be clear, the WHO has absolutely no legal authority to dictate law in the US. Implied consent would have to be legal in the country, state, and county that this action was taking place. They say as much in the document you linked:

Implied consent procedures are common practice in many countries. However, when children present for vaccination unaccompanied by their parents, it is challenging to determine whether parents indeed provided consent. Therefore, countries are encouraged to adopt procedures that ensure that parents have been informed and agreed to the vaccination.

I'm not sure exactly what authority the "SJC Liberty Coalition" would have to dictate or legalize an implied consent doctrine over a school district, so I am skeptical that the notice released, even if it was truly released, has any actual authority to claim what it does. They appear to be a grassroots political group unaffiliated with the government or the school.

I am not aware of any situation in which children are being vaccinated at schools without parents present or written parental consent. I'd be curious if you can find something that is actually from some legal authority (the school, the government, etc) that claims this practice is actually going on in this country.