(1) When assigned, a school crossing guard shall be stationed at a school crossing during time periods established jointly by the superintendent of the school district and the head of the law enforcement agency having immediate jurisdiction.

  (2) While on duty, a school crossing guard shall wear an outer vest of a color and design which conforms with the standards of the manual of uniform traffic control devices provided for in section 608.

  (3) A school crossing guard while on duty at a school crossing shall when necessary stop vehicular traffic. This shall be done by use of a hand held stop sign which conforms to the standards for the sign in the manual of uniform traffic control devices or as approved by the department of state highways and transportation. School crossing guards shall have the authority only at their assigned crossings and only during their assigned duty times.