(a) Except as otherwise provided in subsection (b) of this Code section, it shall be unlawful to operate:

(1) A school bus transporting school children to and from school or to and from school activities at a speed greater than 40 miles per hour on a public road other than one which is a part of The Dwight D. Eisenhower System of Interstate and Defense Highways; or

(2) A school bus transporting school children to and from school or to and from school activities on a public road which is a part of The Dwight D. Eisenhower System of Interstate and Defense Highways at a speed greater than 55 miles per hour.

(b) When a school bus is transporting school children to or from an event or school activity or an express bus transporting students from one school to another school and is not loading or unloading children during such transportation, the speed limit shall be 55 miles per hour on other public roads as well as on those public roads which are a part of The Dwight D. Eisenhower System of Interstate and Defense Highways.