We will be visiting Spain for this Xmas and New Year's. My two daughters are college students in the US, one of which is doing her semester abroad in Paris. Will they be able to get student discounts by showing their US college ID's? At least my daughter in Paris that has an ID of the organization in charge of her program in Paris, "IES Abroad"? In other words, what would happen if we buy student tickets to some place like Sagrada Familia or La Alhambra with these US student IDs?

Sagrada Familia has the same discount for age less than 30 as student, so probably doesn't matter, does it? Alhambra doesn't have a. student discount, they have one for Carnet Joven but that is only for EU residents, same for Prado. Casa Battlo will probably work ok. So it depends. I don't think a card from a tour organizer matters, the IES thing.

When my daughter travelled in Europe she needed an ISIC card, applied for online. I don’t think she used it much though.

When I was a student travelling overseas for research (some years ago now), I bought an ISIC card: https://www.isic.org

This is an internationally recognised student card and should get you the student discounts everywhere you go. It is not particularly expensive, and works when your country doesn't recognise students from a given country. I used mine quite a bit for galleries and museums.

Lavandula

ISIC is an option but I would first check the ticket information of the individual sites they want to visit to see if they might already qualify for discounts because of their age. They might be able to get away with simply showing their passport for a discount in some places without needing to show their student IDs or other membership(s).

I don't think the ISIC card has any value these days. It did when I was a student, long ago, but already when my daughters were students, it was not worth much. Now my granddaughter is a student...

As mentioned, check the sites individually cause there's no "general rule" regarding this. In any case, remember that most US-issued documents have little or no legal value abroad. In Europe for example, that is the case, unless accompanied by an internationally recognised document, which sometimes will replace it altogether, ie. above mentioned ISIC card (or if there's a specific bilateral agreement in place). This applies, for example, to US state-issued driving licenses.