i have two "formal" offers and one e-mail / phone offer from 3 schools. none of them have explicitly mentioned the apr 15 deadline -- unless maybe I'm not looking carefully enough -- but all have acknowledged it one way or another. you'll probably get a number of materials from school A in the coming weeks, some from the dept., some from the grad college. they move at different speeds, and sometimes aren't aware of what the other's doing. i'm sure you know that, though. from one school, i have a formal offer letter from the dept., one from the grad school, another with a TA contract, and a formal e-mail from the president of the school. i'm just sticking it all in a file so i can keep track of it.
just hang in there! again, if you are certain they are a member of CGS, they cannot force an answer. if it will ease your mind, i'm sure you could find someone in the grad admissions office who will confirm this.