Hi Karin, I really liked your take on this whole school thing, I'm amazed you want to put such a huge effort in it.
And please appreciate your project is a lot to take and it can also be a lot to do: the risk is going beyond the assignment for the sake of exploring technologies, architecture styles, desing patterns, and so on, just for the sake of it (don't take me wrong, I've done it all my life).
So my first advice, as quite an experienced software developer, is always go back to some fundamental principles before starting anything, before thinking to anything, and perhaps the most important principle of all is KISS: Keep It Simple and Short (search it up, there are a few versions of what those letters represent :) ). In your case, since it's a school assignment, simplicity is even more important because it makes things clear which makes learning so much more effective.
When dealing with complex tasks like this, another fundamental element is focus: where should you keep your attention when designing and then developing a software product?
In this specific case, I lack what the original assignment was requesting, but I'm quite sure the point (or one of the points) was to make you think and then act on something that didn't require months to be developed, it was to make you learn how to accomplish a task without getting lost in details or in a project too big to be finished in a finite time.
I may be wrong, but I'll keep this in mind when writing the below lines.
FIrst, the architecture of the software product looks like a classic three tiered one: frontend, backend, database. Keep in mind another fundamental principle here: the separation of concerns, which leads to different decoupled architectural elements. Also, just for clarity, the frontend(s) will talk only with the backend, while the backend will talk with the database: this will help you isolate the database from the frontend, ideally enabling you to change database technology if needed.
Second, you explained you want to go web and mobile for the frontend tier: this inevitably will lead you to the conclusions you pointed out correctly, having to choose a number of platforms and languages to basically create the same application, but the fragmentation of different knowledge and procedures can make your life quite complicated and probably miserable.
Personally I'd go for native Android and React for web. Recently, though, I stumbled upon Flutter which, through the same codebase (in Dart, very similar to JavaScript) can create for you applications for mobile (Android and iOS) and the web: I tried it and I've been blown away by the effectiveness and easiness of using it.
For the backend, keep playing with Flask and build a RESTful API, all in all python is a language way more readable and maintainable than JavaScript, and with node.js is so easy to fall into the callback hell (recently less and less but still). Stay away from Java and its ecosystem if you want to finish you project at all (just kidding).
On the database tier, remember NoSQL databases can be quite powerful, but in your case try something very simple (redis can do), or just go with MongoDB as it makes easy to start and evolve your data structures. If you're more the structured type and you want to go RDBMS, try postgresql, it's easy to start (it has also NoSQL features) but so much more powerful and you could learn real SQL on it (stay away from the omnipresent MySQL, it's kind of odd sometimes).
I hope the above didn't sound too much of a lecture, and I also really hope you learn the most important lesson of all: always keep in mind the big picture!