In that case you would not have an effective mediation. One of the first things you have to do in a mediation is settle some of the classic things like whether you have the authority to settle, or whether you agree that it should be confidential or public. How much time do you have? What would be the outcome? Do you want it as an order, a decision, a private agreement?
These kinds of things are usually settled in advance of going to the actual mediation. Where there's a massive disagreement of essential items, then the mediation doesn't take place and it goes to the next stage, which is an official case.