No legal advice here, just suggestions.
Did you keep a copy of that cashier's check? Do you have a bank statement with a copy of it? Can you back provide you with a copy . . . Sounds like it was obviously made out to the mgmt people.
Do you have a copy of any written agreements with your roommmates?
Keep in mind, that if you agreed to 12 months and are leaving in 10 months, most likely YOU ARE NOT entitled to the desposit, as you are breaking your agreement and your rental fee now becomes your roommates responsibility.
My non-legal opinon? No lease? You are jammed. You might want to read your roommates' lease. You may find that is says that they are jointly as well as individually responsible. What that means is that even if one of them moves out, the mgmt does not have to give back their share of deposit.