Who stole Darthin's hand? Solution:

The hand was hidden behind something that was very heavy, too heavy for Dr. Frankenhammer to move on his own. It took an Ogre to move it. Therefore, the culprit could not have been Jud, because as the clues say, he's a normal human boy when in the classroom.

That leaves Frankie and Frieda, who are both strong enough to move the microscope. The second clue was the powder on the floor. It had been swooshed all over the place. The reason for this is that the Hand Stealing Thief had left footprints through the powder that would identify him/her. Two of Frankie's footprints had not been swished away. If Frankie had stolen the hand, he would have swooshed all of the footprints away, but it wasn't him. It was... Frieda. 

Frieda thought that if the hand went missing, she could find it for extra credit. 

Half-credit will be given to those who chose Dr. Pravus. He's supremely evil and cunning, and we at Dr. Critchlore's School would never put anything past him.