Mass. Gen. Laws ch. 85, § 21A

Subject to the provisions of section two, cities and towns are hereby authorized to erect and maintain on state and town and city highways such warning signs, lights or markings as are necessary for the protection of school children; provided, however, that, subject to compliance with the department's specifications and standards, figures or objects may be placed or erected on such ways by cities and towns at school crossings for the protection of school children without the approval of the department.