I am shopping for a MBP for myself to bring to school next year, so naturally I researched the student discount.
-At the store, I brought a copy of my acceptance letter (I am still a high school senior) and they were okay with that, ready for me to check out.
-Using the online store, they ask for an email address, which I won't be getting until May or June, but I could always use my brother's email since I know he won't buy a new apple (or any apple for that matter) every school year.
-Because I don't have an email address, I called up apple on the phone. All they wanted was for me to tell them I was accepted to my school (Penn State) and they would give me the discount.
In summary, any of the three buying option are very easy to use the discount with. Apple works on the honor system (kinda like with the software licenses) so you should be all set, especially if you already have a student ID.