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 shall be 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 such matters would use in the conduct of an enterprise of a like character and with like aims.