Hi, you could probably buy unglazed cheap tiles from a stoke supplier. If you limit your colour palette and find some good victorian examples to work from the pieces could look quite effective. You could make/ get the pupils to make some simple stencils using newsprint paper which could create some interesting repeat patterns? not sure about the paint, you could test out the squirty paint first. Low fired tiles will be quite porus. you could get unglazed vitrified tiles which will be non porus. PVA will give a gloss and is not toxic to use but might smudge water based paint. you could get a can of gloss varnish instead making sure you use it outside. Is it possible to collaborate with another local school / secondary school that has a kiln? it might take time to organise but the other school might be up for it?