Having gathered machines for a school club I'm going to suggest that you purchase machines. Having a mixed bag of vintage machines takes a lot of time and effort that you could better put into helping the kids with the actual sewing. Also, the kids won't have to learn threading and bobbin placement and winding for several different machines. You could perhaps ask for monetary donations. Your local quilt guild might help that way. Also, a dealer or store might give you a discount or even sell at their price because of where they will be used.