Yu3Er2 - Ask your school if they have a letter explaining the tooth collecting process that you could give to the clinics to make it look "official." If they don't, then just go prepared with a jar filled with whatever solution the school recommends and clearly labeled with your name, phone number, school, contents - extracted teeth, etc. Also, you could give them a letter from you describing who you are and why you want the teeth (I am a dental student at .... and I am trying to get my tooth collection started for pre-clinical classes such as ... and would appreciated if you would help in my education....). Get the idea? Just be professional about it.
ehop24 - I don't know what society you ended up joining, but part of the welcome package as an ASDA member includes the tooth collecting kit.