* Easy: ask your dept for a job or RAship to continue to support and build the app and grow its utility. We pay students all the time to build website infra for us in the cs dept. It is usually a ~20$/hr rate, though, so you won't make too much, but it will fund your improvements.
* If you built the app on your own laptop, and you've published the app to the app stores on your own account, you likely own all the rights. Good for you. If the app is hosted in anyway on university resources, then you should be careful if you want to grow this into a business.
* From my own experience, it is very difficult to sell to individual university departments. Much easier to get hired as a consultant. But "enterprise software" that is sold by a license may require the "university procurement" people to get involved. They will went some evidence you'll be around for 3-5yrs, etc...
* Look into how Piazza, Gradescope, Overleaf are doing with university-wide account sales. From what I know, those sales processes are slow and expensive, even though the apps bring enormous value to faculty (e.g., i end up paying for my own single accounts and expensing them against my research budget). Use your network to find someone at those companies to do more research.
Good luck!
btw, happy to try your app and give you feedback