Having to do a demo is a pretty standard thing here at schools.
It’s actually not good advice to have the contract signed before you get here, because you might get to the school and find out things you didn’t know, and not have a good feeling about it at all. I know that you feel much more secure getting a job and having a contract before coming, that is what I did. But now that I have been here a while and worked, and seen schools, I know it is a much better idea to visit the school and meet the managers first before signing a contract.
Good luck. I hope you find a good job. Unfortunately, I have found it’s pretty hard to weed out the bad ones and find a good one.