My family of 4 (including my 20-year-old daughters who are currently university students in the US) are traveling to Paris in May. I purchased tickets to the Eiffel Tower (2 adults and 2 students). As proof of age and status for my daughters, will the authorities at the Eiffel Tower (and other sites) accept their passports and American university IDs, or do we have to go online and purchase international student IDs that Rick Steves talks about in his Paris guidebook?
The Eiffel ticket option that I see is for a Youth aged 12-24. So their passports are their qualifying ID and student status doesn't matter. The same is true for museums like the Musee d'Orsay which has a discount for 18-25 year olds who are not citizens or long-term residents of an EU member state (and free entry for the same age group if you live there).
At the Eiffel, your daughters will be using a YOUTH ticket (age 12-24) and not a 'student' ticket. They should use their passports for proof of identity/age, as they should anywhere else ID is required.
Conditions of ticket purchase may be different at different attractions so you'd need to check them individually. For instance, youth discounts for ages 18-25 at the Louvre apply only to residents of the E.U. But if you're going to be in Paris on a Friday, anyone under age 26 of any citizenship/residency can go to the Louvre for free after 6:00 PM.
Their student IDs may help at some places. Be sure to bring them and ask if there is a student discount when purchasing tickets. Sometimes there will be a youth discount and sometimes there won't be any, but it doesn't hurt to ask.
The worst they can say is no (and berate you for not living in an EU country - happened to me several times in Italy).