Software is usually sold on an as-is basis. If it doesn't quite do everything you need it it to, then either you bought the wrong software or you have to change what you need. Software is not often sold on the understanding that if you find an error or you need it to do something else or to do something in a different way that you can change it, or pay someone else to change it for you. Typically if you're unhappy with your software for whatever reason there's nothing you can do to make it better.
Open source software changes all of this. "Open source" means you have access to the source code of the software. You have permission to change the software and correct the things that are bothering you, or add new features, or take features away. And if you don't have the skills to change these things, you have permission to get other people to fix them for you. You have permission to make copies of your changed software and install them wherever you need them. In fact, so long as you comply to some fairly simple requirements, you even have permission to spread your new copies of the software far and wide.
Open source software is much more customisable than propriety software because ultimately, if you don't like it, you can change it. In some cases changing the software may not be practical, but in many cases whole systems can be put together in ways that match the specific requirements of your school just by gluing together existing open source packages.