I'm afraid I'm not familar with any products to make items safe for your aquarium. I did a Google search and it found nothing, so I'm fairly certain they don't exist.
Generally terracotta pots and mugs are quite safe for use in an aquarium. I believe mugs are just ceramic with glaze? So if you plan on creating custom caves and things out of ceramic for your tanks - I would suggest glazing them. My rough understand of ceramics is that to finalise any item you glaze and cook it?
Would love to see what you end up making, I may try and have a crack at it myself in future. Custom aquarium caves and items sound like a niche item one could sell on eBay!
I wonder as well. I have wanted to create my own decor--I keep hearing that you can't, but then you see stuff in pet stores sold for aquariums--what are our decorations made of then? I would like to work with that stuff! I have the money and can get access to the tools needed to create 'real' stuff like that, not just sticking together things around the house.
They feel like some sort of clay. You know the decor I'm talking about, for hides and fake ships and stuff! Some sort of ceramic thing. I know there are ways to seal things(I have built things for geckos before out of styrofoam and sealed it with grout/concrete, became completely water proof!) so I guess it's just a matter of finding the right tools.
child safe enamel paints used on non-glazed terra-cotta or other ceramics can be considered aquarium safe... the paint WILL come off after a while.
Generally the child-safe glazes are ok for use in aquariums but the glazes can be as hard or harder than the glass of the tank. Your class instructor should be able to find straight dye powder, same stuff as used in the glaze, which can be mixed into a thin clay slurry and applied just like glaze.
No idea if the teacher will allow it.
The basic items you'll be making are actually aquarium safe, kilned grey clay is softer than glass. Keep internal cavities and air-bubbles in mind, they can end up harboring bacteria and sometimes become pressurized enough to crack pieces that even survived air bubbles in the kiln.