How to separate authorization from business logic?

Question

C++ / C#

Hello, I am facing the following issue. I want to make a test system so that teachers can create tests, and students can solve them. I'm trying to implement this as a separate library, so that at the output all work with the service happens through one interface. For all this, one database is created, and it turns out that in the same database, in addition to user identifiers, we must store their roles. This means that we must implement authentication inside the service, which will most likely lead to data duplication, when used, for example, with asp .net. Please tell me how to organize it beautifully.