Hello,
I have experience in teaching Scratch and AI2 in school so I can do this project.
I suggest using Firebase Database. It's very intuitive platform from Google.
It's for free if 1GB stored limit and 10GB/month download limit is enough for you. If you want more, then you can pay 5$ for extra GB stored and 1$ for extra GB downloaded.
There is Firebase component in AI2.
I can make this app in AI2 according to your description and how to use tutorial in 4 days.
Best regards,
Janek