The ISIC is the only proof of student status that is recognized worldwide (in more than 130 countries). It is issued by a non-profit organization called ISIC Association and was officially recognized by the UNESCO as international student ID card in 1968.
Authorized issuer in Germany is the rds.
Eligible are students as well as high school students who are at least 12 years old.
The ISIC costs €15 and has an internationally regulated validity of one year from the date of issue.
The website www.isic.de provides information on and links to national and international ISIC partners in the areas of travel, transport and accommodation, culture and entertainment, sports, shopping and gastronomy. The website also promotes discounted online offers for hardware and software as well as reservation and booking tools.

The easiest way to apply for the ISIC is online at
https://www.isic.de/de/isic-beantragen/