Even though the outside looked and felt dry, there was still moisture present inside...enough to create steam and explode outwards destroying the pieces.
Unless you live in a very arid place like the Arizona or Texas, a piece like a bust would need to dry for months to get bone dry because of the humidity in your studio.
Which if you are mopping and wet cleaning like a good teacher would insist on for safety it would be.
Unlike smaller work where you can take almost dry greenware and dry it in a low heat kiln, large pieces like the portrait busts need to be absolutely bone dry before they are fired. They also need to get to bone dry slowly (partially covered with plastic). If dried too quickly the outside will shrink more rapidly than the inside and the surface will crack.
As the clay moves from leather hard to bone dry, the color of the clay will change ... it will become chalky and will not be cool to the touch.
Touch the inside of the piece too where you hollowed it out to test to see if it feels cool. If it does, wait.
Always err on the side of caution when drying large pieces. If in doubt, wait.
Another suggestion is to try using P-clay (Paper clay) for your students next portrait bust project.
The paper fibers in the clay body allow the pieces to dry more evenly... they wick moisture from the inside to the outside.
You can also force/speed the drying process without being afraid of the outside cracking.
Heather in Joplin