Any person employed in a public community college upon a full-time basis and who shall be duly certificated under the law governing the certification of teachers, or any person employed in said public community college in a full-time teaching, supervisory or educational administrative position certified by the executive officer of the institution for such full-time duties shall be a member of the public school retirement system of Missouri; except that, if the employee is a member of the public education employees’ retirement system at the time such employment would qualify him for membership under this section, he may elect to continue in that system if he makes the election to continue at the time he becomes eligible for membership in the public school retirement system.