I received a PhD acceptance (with fellowship) via informal email from my proposed supervisor on March 6th, received the paper letter in the mail on March 12th, and was asked to reply whether I would accept or not no later than March 13th. I called and spoke to the administrative assistant to ask if I could have an extension, because I have only heard from three of my five schools, and she said I could let them know in a week. But this seems a bit rushed to me. Is this an attempt at this school to secure students as quickly as possible? I also thought that I would have until April 15th.