In several countries (e.g. Egypt and India) many venues (e.g. museums) have substantial discounts for "students". How carefully the student status is checked varies greatly, but at many places the guard insists on being shown a Student ID card with picture. Lacking that, the teenagers must pay as adults.
My three daughters are teenagers (ages 14, 15 and 17), two of them attend primary school (grundskolan) and one attend high school (gymnasiet). So, as of yet, none of them study at a university.
Can I get some Student ID card that will be accepted internationally?
I have not been able to find anything myself. Primary school is compulsory by law in Sweden, so strictly speaking it is redundant to require any student id for Swedish children of primary school age, but this argument is rarely accepted. Passports proving their age does not help.
ISIC requires attendance at a university.
Studentkortet is the closest I have found. It is an app that can be registered for high school students, but this has been refused on several occasions on the basis that it lacks a picture (the fact that it is an app probably did not help either).
I haven't found anything for primary school pupils.
(It is of course entirely possible that these guards are just running a racket where they profit by being overly zealous about this rule, but it would be much easier to argue with them if we had something resembling a real card)