Hey and thank you very much of the suggestions, much appreciated.
I know of Octave. Years ago I had plans with a mathematics teacher to do my thesis of Octave as a replacement for Matlab.
Now we talked about GNU Octave too, but then I found Sage, which is a very interesting alternative in mathematical analysis and what not in schools because of the web browser interface, server/client model, feature to share and publish worksheets, individual user accounts with disk space for worksheets, and because Sage uses Python by itself.
Sage has gathered and united nearly 100 Mathematical Free Software programs under one and same user interface, which means you can in theory do tons of stuff by just installing one package. Among those are GNU Octave, so you can actually use Octave from Sage, if there is a reason for that.
My job is now to install and configure Sage for the school and educate the teachers how to use Sage. Next month I will have to get a new job xD So I am not maintaining Sage on the school, and that's why I want to make them a system which is running without problems after 5 years. Thus upgrading the OS after a year or so is not a choise.
