If you want to guarntee playability use something like Cinepak and render to a quicktime movie. Cinepak, though not the greatest codec by any means, is pretty usniversal. It was part of quicktime 1.0 (and even the beta, which I still have on floppies!)
Make sure your total data rate is around 1000k/sec.
If the machine are iMac you could probably bump up to using the sorenson compressor (codec). With sorenson you can use a data rate around 200k/sec.
Note make either of these 320x240 for max compatibility with old hardware.
You'll want to encode to Quicktime, however you'll want to verify the version of Quicktime on the Macs at the school as you'll be encoding to Quicktime 6 with Vegas.
You can download Windows Media Player for Mac however some schools do not allow access to external web or even internal hard drives. This you may want to verify with the teacher.
You'll see lots of different suggestions listed here. Do ALL of them! Use a variety of file formats, codecs, frame sizes, and bitrates. Put as many of them as you can on the CD. That way once your son gets to school he can try them to see which plays best. Whenever i send a short clip somewhere i usually include at least half a dozen different variations just to make sure that at least one of them will be playable on the target computer.
Also make sure the CD itself is made as standard as possible. I'm thinking filename length, Joilet, ISO 9660 and all that. No funny burns. Maybe burn something for him to try out on the Mac before the big day.
Best/Tommy