A lot of museums and sights have youth or student discounts. Do you need some sort of international card to prove that you are underage or a student? Or can you just show an ID from an American university?
Thanks,
Adam

I think it depends on the museums you are visiting. Some may accept your student card from your country but some others may require an ISIC card. To be on the safe side, I'd get the ISIC card. If you can get discounts for students somewhere, this card proves you are a student everywhere.

Most museums we visited this summer were content to check the student's passport to see if s/he was really under 18. In some cases, when the student was really pretty young (13 or so), we were just waved in and didn't have to show anything. We found it paid to ask...some places let students in for free!

I don't know what the policy is for students over 18.

I got into the Prado and the Reina Sofia with a student discount using my college ID. Let's just say that I am obviously well over 18 (or 21 for that matter), but they had no problem giving me the discount. and I AM actually a college student.

I'm a grad student, but they wouldn't let me use my ID for the Prado because it doesn't have a date (the only date is the date of issue--sometime in 1995--they probably thought I was trying to pull a fast one). I got the impression it would have been fine with a current school year mark on it.