Though the town owns the school building, it is not a place for public business. Since the story in the newspaper did not say that any the members of the group were either a teacher, student, janitor, cook, parent, relative or friend of any of the students they had no business being in that building at that time. They were not invited in, nor did they ask permission from the principal to be there - that is trespassing and that is against the law - period.
For young children school is usually the first major exception to their parents' rules. "Do not talk to strangers", "stay away from people you do not know", "do not eat food given to you by a stranger" and "stay close to me" are the things that parents start drilling into their young child's head. At age five these children are sent into a building alone and are told to listen to, obey and eat the food served to them by strangers! Though these people are teachers, principals, janitors and cooks; they are strangers to the children on day one. Thus for anyone to go into a school building and do their own thing is violating a trust that parents have with the school system.
For the town not to prosecute a dangerous precedent will be set in these times of our precedent based court decisions. The first amendment "right" will supersede the need to keep our school buildings secure in the courts of precedence (not law). This would become an implied invitation to groups.