I have known public studios to require cookies, the same as stated above. One of them requires the students to kiln wash the cookies. This allows them to reuse the cookies even when the glazes runs, most of the time. A recipe for kiln wash that I like and use, that also makes a usable wadding if thick enough,
1 part Silica
1 part alumina
1 part Kaolin (any)
2 to 3 % bentonite
Tom