Just found this thread.
I am actually in the process of making (for my own amusement) some McD character heads. The characters have been a love of mine since I was a child and the fact that they are Sid & Marty Krofft creations (my heroes) makes them all the more special.
I don't see the harm in producing props for a school project and as long as you are not making them to sell it SHOULD be ok (morally). I don't know much about the law regarding these matters, but I would imagine if you didn't go shouting and advertising what you are doing you might not need to worry so much.
BTW, I have come across suppossed costume pieces for the characters on eBay from time to time. You might want to check. There's a Hamburglar mask up there right now.
I'd love to see the results if you finish this project!