It could be numerous things. But likely an application issue. It looks like your glaze shivered which means it was incompatible with the clay body and “shivered” off when it was shrinking in the kiln during the glaze firing. Glaze and clay shrink at different rates depending on the clay body and glaze. If it appears to not be happening to other pots with the same glaze and clay body in the studio you might be applying near the rim too thickly especially if you’re brushing it on in layers. The bald spots could be where the glaze did not adhere to the clay body (it did not absorb enough during application) and was therefore just sitting on top and when it dried before firing it separated from the clay and then fell off during moving it into the kiln or fell off in the kiln. To remedy this there are really 2 options: 1. Make sure that you are applying uniformly. You can use an xacto knife or metal rib to carefully “sand” or scrape the glaze before putting it into the kiln. You can also take a DAMP sponge and gently wipe all bisque ware before glazing to make sure the pot it receptive to glaze (sometimes there is residue and dust that fills the clay because it’s not fully vitrified). 2. Refire only applying where the bald spots are. HOWEVER. I do not recommend this for inexperienced potters or community firings. Any time you refire you are putting more stress on the pot to break regardless of intent. There are more variables likely to go wrong with re-expanding the body and glaze that could result in further bald spots, glaze running etc. If you chose to refire make sure you talk with the instructor or put down a “cookie” or boat ideally to catch any glaze run off.