The school used photos of all the children for publicity and forwarded them to friends. Without the consent of the child's guardian before, is this a violation of the right to portrait?

This is an infringement of the right to portrait.

Relevant laws and regulations for reference:

Portrait right is a kind of personality right enjoyed by natural persons, and its content is the personal interests embodied in their portraits. A work that reflects the image of a natural person, including the five senses, through photography or plastic arts. Portrait right is a kind of personality right, the right of natural person to make portraits and the right to use standard forms. Legal portrait is an integral part of the personality of a natural person, and the spiritual characteristics embodied in the portrait can transform or derive the material interests of citizens to a certain extent. The legal protection of citizens' portrait right is based on the portrait right, which reflects citizens' spiritual interests and personality interests in many ways. According to the Law of People's Republic of China (PRC) on Portrait Rights, the right to portrait is a basic right of citizens, and no one may use or insult his portrait without his consent. The police have the right to retain the right to use. If you use the portrait of the victim without authorization, you can negotiate first. Those who refuse to revoke it may sue according to law, apply for judicial protection and safeguard their legitimate rights and interests.