Ditch the screensavers.
Using screensavers wastes energy and money. Make sure that when not in use your school's computers default to standby mode and that they are turned off at night.
If it's hard to get the person in charge of the computer lab to change the school's policy, remind them that the school can save up to $75 or more per computer by activating system standby or hibernate features.
Go to Energy Star's FAQs on computer power management http://www.energystar.gov/index.cfm?c=power_mgt.pr_power_mgt_faq for more information and learn more about the environmental impact of computers at NRDC's SimpleSteps.org.