“Employment” does not include service performed by an individual under the age of 22 who is enrolled at a nonprofit or public educational institution which normally maintains a regular faculty and curriculum and normally has a regularly organized body of students in attendance at the place where its educational activities are carried on as a student in a full-time program, taken for credit at such institution, which combines academic instruction with work experience, if such service is an integral part of such program, and such institution has so certified to the employer, except that this section shall not apply to service performed in a program established for or on behalf of an employer or group of employers.