We also make birdfeeders using old bottles, it takes only a few minutes and we try out a few different types to see which one works best. Why not try them yourselves at home. It's more fun than unnecessarily spending money buying new ones. When they get too dirty then can just be rinsed out and put in your green bags to be recycled so it fits in with the school ethos.
Of course whilst it is important to make sure that the bird tables and feeders are kept nice and clean so the birds to not get infections; it is always important that we remember to properly wash our own hands after we have been in the garden.
You can find some instructions on the RSPB website for children here. Every year we try and take part in the RSPB's Big Bird watch and you can see the photos we have for 2010 and 2008 just click on the photos.