Can you keep a tadpole from changing into a frog?

My science teacher had 6 bullfrog tadpoles for our class. 3 changed into frogs, and he said he was able to keep the others as tadpoles. He hinted that he put something in the water to control this, but won't tell us what yet. Any ideas? Oh, and 2 of the "controlled" tadpoles died because they developed internal cysts.