School Idol won't load on tablet, and what is "rooting"?

"Rooting" means modifying the device's software to give the user superuser ("administrator" in Windows parlance) access. Most devices do not allow this out of the box, but some budget devices are sold already rooted. So my guess is that your tablet is one of these.

Unfortunately I don't know how to solve your problem: if you've got past the root checking (the "illegal access" message) and it still doesn't work it might be incompatible with the device, but it's hard to say. If you could tell us what the error was when you tried to install xposed it might be that someone could suggest a solution to that - but unless you register you can't reply to this thread, though you could post another question if you had the information.

 

Thanks for the reply Turns out I already have a proper account under a different name. Sorry for the confusion, and my delayed response.

Here's what I get when I try to install Xposed:

 

I don't know how the Xposed installer works (never tried it myself), so can only make guesses. First thing would be to try a root checker app to see whether you really are rooted (or try installing and running Titanium Backup - that will tell you if there is a problem with root). If that all works, I wonder whether installing Xposed needs a custom recovery? But the message you have there suggests that's not the problem.