Opinion on accepting a grad school offer after having already accepted another offer I applied to graduate schools this year, and at the tail end of February, my first pick school sent me an email that to me, sounded like they had filled all the available TA positions. So naturally I chose my second pick school (that did offer me a TAship).

Today (April 16th), I receive an email from my first choice school saying they would like to offer me a TAship (apparently somebody declined their offer).

The graduate adviser at my second choice school had told me before that sometime she made offers to candidates in the summer, so I am thinking if I were to contact her and ask to be released from my agreement, she might be OK with that. (I realize there are formal rules that apply)

Would it be such a bad thing to ask my second choice school to release me (if they say no it will be somewhat awkward for me and if I just walk out on them, I am not sure what they can do).

TIA,
GB
You have to know what you want to do. If you want to go to your 1st choice school, perhaps you can come to an agreement to have the 2nd professor as your copromotor? Or didn't you pin down your supervisor and your topic yet. In that case, just go for the first school definately. 

Take a look at your contract or contact the administration. They will be able to help you with the more technical details of resigning. 

But do what you feel you want to spend the next 4 years doing!