is the international student ID really needed?

thank you!

FYI, some places will require proof of birthdate for student discounts. I pre-bought student tix for my 16 year old for our trip last month for the Uffizi and Accademia. Uffizi pickup office just looked at him and asked his age-no ID required. Accademia required proof of birthday--high school student ID was not sufficient. Thankfully I was able to pull up his health record online and show it to avoid paying the difference as we didn't have passport on us.

I'm only guessing here, but many sights have both a youth discount (which might cap out at 18 years old, though I assume it varies) and a student discount. If a visitor is 19 or older and is hoping for tickets at the student rate, I think he or she may well need a student ID card in some cases.

Just to note:
At Italian STATE museums, children 17 and under are almost always admitted for free, although some of them require the same advance reservations as are required for adults, and there may or may not be a fee for guided tours.

When those museums state that discounts are available only to young people 18 - 24 of European Citizenship, or other listed European citizenship, they usually mean it so an ISIC for an American wouldn't suffice. An example from the Uffizi website:

https://www.uffizi.it/en/pages/free-and-reduced-tickets#reduced

As well, students older than 17 are only offered free admission under limited circumstances, and an ISIC doesn't apply:
https://www.uffizi.it/en/pages/free-admission-teachers-students

Teachers and undergraduate, PhD, MA or post-doc students enrolled in
the European Union's Faculties of Architecture, Cultural Heritage,
Conservation and Educational Services, as well as Literature and
Philosophy (Arts and Humanities) only if enrolled in literature
degrees with an archaeological or historic-artistic specialization.
Students are requested to show a valid certificate of enrollment for
the current academic year, in Italian or in English (for foreign
students). NO International Student Identity Cards are accepted.

So if your young person is under the age of 18 but may look older, use their passport...which, according to Italian law, should be carried by you or by them anyway. Discounts at civic or private museums may vary so you'd have to check them individually. For instance, I see that the Vatican Museums offer a student discount "for students up to 25 years of age in possession of a document issued by the school or university institute that proves registration. " They do not dictate that the student's ID be an ISIC.

https://m.museivaticani.va/content/museivaticani-mobile/en/organizza-visita/tariffe-e-biglietti.html

So bring your young person's U.S. student ID but it I don't know if you need to bother with a ISIC. Do you have a list of attractions you wish to see and so can check them all for requirements?

The ISIC isn’t necessary if the student has an ID from their school—or some other form of ID proving their age. I travel with students all the time and gave up long ago on the ISIC because it wasn’t useful. I concur with what Kathy intimates: that many discounts for major attractions are for European Union citizens, so when you see the discount listed for “student” it often does not apply to US students.