I don’t think I’ve actually heard “in” used there
You have, many times. Trust me.
“I’m a graduate student in philosophy”.
-> “I went to graduate school in philosophy”
-> “Philosophy? I went to graduate school in it.”
The Googles would seem to agree:
http://www.google.com/search?q=%22graduate+school+in+philosophy%22
http://www.google.com/search?q=%22graduate+school+for+philosophy%22
You have, many times. Trust me.
“I’m a graduate student in philosophy”.
-> “I went to graduate school in philosophy”
-> “Philosophy? I went to graduate school in it.”
The Googles would seem to agree:
http://www.google.com/search?q=%22graduate+school+in+philosophy%22
http://www.google.com/search?q=%22graduate+school+for+philosophy%22