How would you deal with a disagreement with an adult in a professional and appropriate manner?
Deal with the person you have the disagreement with first; be assertive and deliver your side of things in a straightforward, factual way – also listen to their side of things and see if you can find a way to compromise. Try to see their side of things and remain impartial. Most of all keep calm. Failing that, try to agree on someone who can mediate i.e. someone who can listen to both sides impartially and help you find some common ground. If that doesn’t work – give ’em a smack on the nose!