The commissioner in establishing such standards shall classify insofar as practicable the fields or categories of administrative, supervisory, teaching or other educational services and the fields or categories of school nursing services which are being performed in the school districts of this state and may, in his discretion, determine seniority upon the basis of years of service and experience within such fields or categories of service as well as in the school system as a whole, or both.
N.J.S. § 18A:28-13