Too many goodsides
Yes, it is true that you can change lots of software under GPL to do something that its intent was not to do but the fact is that open source software licenses have too many goodsider. If there was a bug in a program or a possible security hole the user can just edit the source code and fix the problem. If the program does not have what the user needs, he/she can fix it to his needs. If a software producer needs any help with the creation of his software, thanks to GPL and other open source software licenses the software producer will be able to hire employees to work on the application source code remotely.
Since Linux/Unix is also a much more stable system, it will be a much better choice than Windows for holding large and always updated databases, such as grades for example.