It's just stupid.... What is the point of putting a motherboard in the dishwasher?? Was your teacher eating off of it?? If there's macaroni and cheese stuck all over it then I can see puting it in. If a mobo is in such bad shape that it warrants a dishwasher....then let's see what happens if you put it in the trash compactor....where it belongs!!
My mobile phone got dropped in a toilet overnight (while it was still switched on) then I just took the battery out dried it all realised it didn't work replaced the battery and all was fine!