In several countries (eg Egypt and India) many venues (eg museums) have significant discounts for "students". How carefully the student status is checked is very different, but in many places the guard insists on showing a student ID with picture. Without it, the young people have to pay as adults.
My three daughters are teenagers (14, 15 and 17 years), two of them attend a primary school (basic college) and another a high school (high school).
So none of them are still studying at a university.
Can I get a student ID that is accepted internationally?
I could not find anything myself. The elementary school is legally compulsory in Sweden. Therefore, strictly speaking, it is not necessary to ask for a student ID for Swedish primary school children. However, this argument is rarely accepted. Passports that prove their age do not help.
ISIC requires a visit to a university.
Studentkortet is the next thing I found. It is an app that can be registered for high school students. However, this was rejected several times because a picture is missing (and it is an app).
I did not find anything for elementary students.
(Of course, it's quite possible that these guards only run a club that benefits from excessive zeal over this rule, but it would be much easier to argue with them if we had something that resembled a real card.