Employees of Washington University in St. Louis can’t maintain a lawsuit over retirement plan management by merely alleging that the plan could have offered cheaper or better-performing investments, the school told a federal appeals court.

The U.S. Court of Appeals for the Eighth Circuit should reject a proposed class action challenging the fees, investment options, and administrative structure of the school’s retirement plan, Washington University said in a brief made public March 15. The plan “unquestionably” offered a wide range...