I`ve read that you need to present student identity card (ISIC) for Shonbrunn Palace student ticket. Where do I get it being from US? Do they really ask for ISIC at most European attractions if my children 15 and 18 years old? Passport is not enough?
Where did you read that an ISIC card is needed at Schonbrunn Palace. In looking at the Schonbrunn website, I see no mention of that. The website simply shows child prices for age 5 to 18.
https://www.schoenbrunn.at/en/tickets-and-prices/all-tickets-tours
If the discount is based on age, then a passport should be all that is needed.
You can get an ISIC student card via https://www.isic.org/
If you follow that web site to the Austrian ISIC page and view the benefits you will see, that there are not many places listed and it is of no use for Schönbrunn Palace.
Last autumn my 18 years old daughter was traveling for 6 weeks through Spain, Portugal and Italy with an Interrail ticket. She has an ISIC card and told me, that the only place she used it was the Royal Palace of Madrid.
Thus, no need to worry about it.
Do your kids already have a school ID?