At the request of a public agency, or as otherwise permitted by the board, any class or classes of positions covered by a retirement system which may be excluded from coverage under the federal system pursuant to paragraph (3) or (5) of Section 218(c) of the Social Security Act, and to which the agreement does not already apply, may be excluded from the agreement at the time it is made applicable to such retirement system; except that such exclusion shall not include any services to which Section 218(c)(3)(B) of the Social Security Act is applicable.