It would be nice to replace all proprietary software with Free Software during an all-night spree, but that's usually not practical.
Get the policy in
Maybe the most important first step is to convince the decision makers that the school/company should be using Free Software.
A policy to use Free software can be phased in. One first step would be to tell the school about Free Software and then suggest they replace any proprietary web browsers with Mozilla Firefox. Or, you could ask that they install OpenOffice.org and that they remove any proprietary offices suites (if present).
Component by component, they can move to Free Software, and when they replace the operating system with a Free Software operating system, the change should be quite easy.