Would you say something to the owner of the dog or the school?
The rules of the school are that well-behaved dogs are allowed in the playground but this dog is a breed known for being aggressive and not good with children and the owner had no control over it.
It was on a lead but she was chatting and it was jumping up at little kids and lurching forward with no muzzle. It's not a banned breed in the UK but massive and typically used as guard dogs.