linux and windows (Score 1)

I recommend installing a distro like ubuntu on the oldest machines and let the rest be for the time being. Kids learn things quickly and in todays world it is almost essential to be familiar with at least windows AND some kind of unix system (ie. linux, os x, freebsd, solaris). Furthermore, doing so will leave a back up in case you are not onsite and there is some problem with the linux machines (user problem).