There are a LOT of glazes to chose from on the market. First of all it is important to make note of the temperature you are firing your kiln at. More than likely in a school setting you are using low fire clay and glazes, which is cone 05/06.
If you try to fire a piece with a mid or high fire glaze on your piece, and you are only going up to 05/06 then your glazes will not mature- or become shiny. If someone tries to fire LOW FIRE clay to HIGH or MID temperatures, then the clay will melt in the kiln - completely destroying your kiln!
Make sure that the glazes you purchase are low fire glazes. Next make sure that they are MAYCO foundations opaque. These glazes are the easiest to manage and give the best results on your students final pieces. The colors are bright and also easy to distribute. I cannot say enough good things about these!!!
After a year or two, some glazes chunk up inside the containers, and are especially hard to stir and distribute if they have pieces of glass or crystals in the glaze to give them. This is not the case with Mayco's foundations. I can leave a bottle on the shelf over the summer, come back and give it a quick shake and it is good to go. No stirring required!!!!!
The colors also work similarly to underglazes where they do not run or mix together, but you do not need to add the clear coat after your students finish painting! This is a glossy glaze. You will need to encourage students 2-3 coats, usually I only tell my students 2 and it is plenty!
When I taught clay at Cheekwood Botanical Garden and Arts Museum for 3 summers, these were the glazes that were in the studio. I absolutely fell in love with them and use them in my own elementary classroom now. I even get them through my clay sells man locally for under $10 a pint!!!! Super reasonable!
So if you are struggling with "What glazes to buy," I highly recommend checking out Mayco's foundations!
Below are some pieces that were glazes using this marvelous product! Happy Glazing!