You should never change applications from others directly, as these changes will be lost when there is an update. The system warns about this when you attempt to remove the write protection in the admin panel for an app that you did not create.
You can safely extend or access outside applications, by "building on them" - here is the documentation:
https://rei3.de/en/docs/builder#building-on-applications
In your case, it sounds like it would make sense to manage students in your own app. Doing this you can still use the contact relation from REI3 Organization for most data and then extend this relation via 1:1 relationships inside your own app. Once you have extend this relation with a 1:1 relationship (like contact_student) you can add many more relationships for things like student_type and so on in your app.
