I used to work for a circuit board assembly plant that made microprocessor based controllers. After the components are placed on the boards and the board was run through a wave solder machine, a sticky coating of rosin is left on the board. These boards were then run thorough commercial dishwashers.
However, the detergent used was not regular dishwasher detergent. It was a special detergent designed for that purpose, a detergent that dissolves the rosin on the board. Board washing after wave soldering was a very common way to clean the circuit boards.
Electrical circuit boards CAN be washed in a dishwasher. So your teacher is not a moron as suggested here. He probably knows more than most of the people answering this thread. LOL!
-Target