The board, upon establishing a retirement system pursuant to this chapter, shall create a retirement board of not more than five members, at least two members of which shall be the elected representatives of the employees, to administer the retirement system, and shall define its powers and duties and the tenure of the members.

All members of the retirement board shall serve without pay.

The retirement board shall determine the eligibility of officers, employees, and their dependents to participation in the system and shall be the sole authority and judge under such ordinances as may be adopted by the board as to the conditions under which persons may be admitted to and continue to receive benefits of any sort under the retirement system, and may modify allowances for service and disability. The determination of the retirement board shall be final and conclusive and shall not be modified or set aside except for fraud or abuse of discretion.

If the district maintains its own retirement fund, the retirement board shall have exclusive control of the administration, investment, and disbursement of the retirement fund. The retirement fund is a trust fund held for the exclusive purposes of providing benefits to members of the retirement system and their survivors and beneficiaries. Investment of the fund is subject to the care, skill, prudence, and diligence under the circumstances then prevailing that a prudent person acting in a like capacity and familiar with these matters would use in the conduct of an enterprise of a like character and with like

aims.