actually you do, in the paperwork you receive in the mail accepting the appointment and a list of other paperwork you sign on I-day stating what happens if you leave early what you owe back etc.
And they have to sign the 2 for 7 paperwork, "they" will come track you down if you don't (i.e. you forgot to turn it in and went to class) it's a nice contract you sign, just like you sign a contract at each promotion saying you either accept or deny your promotion and you get sworn in all over again