I think that the analogy between checking for headlice and forcing kids to pee in a cup to find out whether they have been smoking pot is very far off. A more accurate analogy would be a parent who checks to see that their teenage daughter's hymen is intact which most people would consider humiliating and abusive if not illegal. When the nurse searches for lice it is not designed to find evidence of wrongdoing nor is it humiliating and degrading. Not only does forced drug testing violate the 4th ammendment barring unwarranted search and seizures but it violates the 5th ammendment because it forces a person to incriminate themselves.
