Section 42. The administrator may establish a co-operative engineer program and may enter into agreements with colleges of recognized standing within the commonwealth, including colleges which have summer programs, which have established a curriculum leading to a degree of bachelor of science in engineering on a co-operative basis, contemplating regularly rotating work activity in the field of engineering and an equal period of classroom training. The administrator may employ persons enrolled as candidates for the degree of bachelor of science in engineering in any such college to serve in the department in the position of student engineer; provided, however, that the position of student engineer shall be in a grade lower than that of junior civil engineer in the department; and provided, further, that at no time shall the number of persons employed in the department as student engineers exceed 8. Upon completion of not less than 2 years of employment as student engineer, a person shall be eligible to apply for the examination for highway engineer intern. No person shall be employed as a student engineer for more than 6 years.