Here's what you should do. Identify your #1 school choice. Have you interviewed there yet? If so, I would hold off and find out when a decision would be made.
If you have not interviewed yet at your #1 choice, I would go ahead with the deposit at the other school. Meanwhile, call your #1 school and ask them when they will start interviews. If they have already started to interview, you can call them and tell them that you have been accepted to another school and request consideration. Explain to them that they are your #1 choice and that you were only using the other school as a backup plan.
Just a few weeks ago, I interviewed at my backup school first, but luckily interviewed at my #1 school a week later. I then got accepted to my backup school first. I proceeded to contact my #1 school and see exactly when a decision would be made and requested the results via email to save time. When the results came in, I was very happy!