When a municipality under the jurisdiction of a county board of education attains a population of 5,000 or more, according to the last decennial or any subsequent federal census, the schools of the municipality may remain under control of the county board by agreement between that board and the city council of the municipality, which agreement shall be expressed in resolutions adopted by and spread upon the minutes of the two authorities. If the municipality does not enter into such an agreement, the control of the school or schools of the territory within the municipality shall be vested in a city board of education, and thereafter the district school tax collected in the city shall be paid over to the custodian of city school funds, and the district school tax collected in the contiguous territory shall be paid over to the custodian of county school funds; provided, that so much of the proceeds of the special school tax collected in the original school tax district as may be required for the retirement of outstanding warrants issued against such tax, including the interest thereon, shall be paid over to the proper official or authority to be used for such purpose.