It shall be unlawful for any appointed superintendent, principal or licensed employee to be paid for any services as such until a written contract has been executed as is provided and required by this chapter. If any school district superintendent shall make any such payment prior to the execution of the contract he shall be civilly liable for the amount thereof, and, in addition, shall be liable upon his bond. If any licensed employee, appointed superintendent or principal shall willfully and without just cause breach his contract and abandon his employment he shall not be entitled to any further salary payments either for services rendered prior to such breach or for services which were thereafter to have been rendered. Nothing in this section, however, shall prevent the employment and payment of substitute teachers without a written contract.