The school board of any school district that provides transportation for pupils to and from the public schools may, pursuant to agreement with the school board of any other school district, provide transportation for pupils of that district to and from activities sponsored by any public school in that district, at times when buses or other conveyances used in such transportation are not needed for transporting pupils of the school district so providing that transportation. In providing such transportation for pupils of another district, the school board shall charge an amount not less than the cost of furnishing that transportation, including a reasonable allowance for depreciation on each vehicle so used.