You probably don't need parent supervision, but it's a good idea as the agency can talk to them and answer any questions. It might be best to get your parents to read the contract before you sign it too, as you are under 18 they might need to sign it as well ?
I don't know about Indonesia, but I think many agency contracts are three years. If you are unhappy and discuss your issues with the agency they should help you/give advice etc. but I don't think it will be easy to get out of it. If both parties are unhappy I'm sure they'll cut their losses and terminate the contract.