Android user authentication help

I've created an app that allows two types of users to login in (students and staff). Students are presented with a 10 question quiz when logged in that returns the score and time taken to complete quiz, this score and time need to be recorded. Staff login should allow staff users to add/remove students and view recorded results.

I have the UIs set up and the quiz works are intended. I need help with the following:

Host for user authentication --> I tried firebase but it would not allow me to use userID and pw for authentication but wanted an email, which will not work with my app, as each student user already has an id and pwd that they need to be using. An email also will not suit the nature of the app. I cannot do the custom authentication as I'm not hosting on a server. Are there any alternatives to firebase that would be good for this? I'm looking at Back4Apps and Cloudboost but not sure?
I know that with these backend app providers that I could record the results into a database, but I'd like for the data to be viewable to staff users at click of button as either a table but preferably an Excel file to allow the ability to filter etc - any suggestions on how to go about this?
Is it worth just hosting a table of user id and pwds online and use this for authentication? (is this even an option?) would this work if 4...5... 50 users tried to login at the same time?...
I'm very new to this, so any suggestions, advice, pointers in right direction would be really useful.

Thanks in advance!