I (15 years old; living in Germany) write software now for ~four years and wondered whether I can release it into the public domain or under some free software license without having to ask somebody. I know that I have to ask my parents for many legal acts and that my father's employer (he is a professional software developer) has some rights about his software. Have I to ask my parents (as for many legal acts), my school (my "employer" :)) or somebody entirely different? I had searched the web very often but I always only find what the legal status of schools are.
In Germany, there is no concept that corresponds directly to public domain. You automatically hold the Urheberrecht (~ copyright) for all creative works that you make, and it can't be given up or transferred (§29 UrhG). The work only enters the Gemeinfreiheit (~public domain) 70 years after your death. You can however license Verwertungsrechte (economic usage rights). When you make creative works in the course of employment (see §43 UrhG), your employer automatically gets the Verwertungsrechte necessary in the context for the work, which is typically an exclusive right (no one else, not even you, can use the work).
Your are not in an employment relationship with your school, so it has no rights to your works and cannot prevent you from publishing them on copyright grounds. When you see advice on the internet to check with your school first, that is U.S.-specific advice.
Since you're still a minor (7–17), you only have limited capacity to enter contracts or legal transactions (bedingte Geschäftsfähigkeit). While you are able to make transactions involving your own means (e.g. buying something with your pocket money) or make transactions that are only to your benefit (such as accepting a gift), other transactions are schwebend unwirksam (~ pending ratification), until your parents agree. The relevant law is in §107 and §108 BGB.
This is a problem with open source licenses. If you publish software under a license, this license is schwebend unwirksam. Someone might start using the software under the license. But then if your parents refuse ratification, the license would be invalid, and everyone would have to stop using your software and destroy any derivative works they made. This is a bit of a problem, especially since open source licenses are otherwise assumed to be irrevocable.
So if you want to offer a license (including open source licenses), please give legal certainty to the recipients of the license and ask your parents for their consent first, possibly even in writing. Per §107 BGB, the license you offer with their consent will be valid.
We would have to look exactly at what happens if a 15 year old publishes their own work under the GPL license. USA and Germany are not identical but similar.
In the USA, you can absolutely publish your work under the GPL. However, you or your legal guardian can cancel this at any time up to and slightly after your 18th birthday. If that doesn’t happen, then everything is fine.
But if you void the GPL license, it just is not valid anymore. That would mean trouble for everyone using it. They suddenly lose their license, and they are committing copyright infringement by using the software. That would be my risk that I take if I use gpl-licensed software that was licensed by a 15 year old. For you, there is no risk.