The success ratio is high, meaning that all of the candidates interviewed are equally qualified for the position and will likely succeed at the job. The selection ratio is low, which means that the number of people who are hired that would fail gets smaller. The validity coefficient is high, so the people that have been hired will be successful at the job. The tests were helpful predictors of the successful candidates for the position.
Ethically, hiring 10% more staff than needed with the hope that someone reneges is immoral. The individual hired is establishing life changes on the word (and invalid contract) of the hiring organization, while the hiring organization knows it is only a contingency offer from the onset. It expects to withdraw the contract with the innocent employee for no perceptible reason. All of the candidates are equally qualified and expected to be equally successful, so what would be the criteria for eliminating one? Eeny-meeny-miney-moe?
The hiring organization should hire only the amount of teachers needed. If there develops a vacancy, then the hiring organization should search among the top ten approved candidates and make a valid job offer. Contingency offers are not helpful to the employee or for the employer, as they do not provide closure for either party. Also, the candidate pool for other employers has been drained, therefore eliminating the best candidates for all employers. This unfairly skews the candidate selection for the other schools, resulting in inferior teachers. The students need the best teachers available.