If you are replacing school computers do consider upgrading to laptops which consume less energy and also allow more flexible working practices across the school site.
A desktop uses an average of 200 W/hour when it is being used. A computer that is on for eight hours a day uses almost 600 kWh and emits 175 kg of CO2 per year. A laptop uses between 50 and 100 W/hour when it is being used, depending on the model. A laptop that is on for eight hours a day uses between 150 and 300 kWh and emits between 44 and 88 kg of CO2 per year.
Additionally, you should ensure that all computers are setup to go into standby mode, within a short (15 minute) period of inactivity.
Can you compare the energy use of your old and new computers? How much energy do you expect to save across the year?