Just repeating what everyone else has said. You can DEEEEEEEFINITELY achieve 90+. Check out
{SAC RANKINGS} -All you need to know about it-: if you're at the top of the cohort, your personal exam performance is the key factor in your score.
Not to say that others at better schools
don't have the advantage over you - just like if they can afford a gazillion resources and tutors in every subject, they're advantaged. But if you start blaming everything on this, and give up without a fight, you're just keeping up that cycle.
So,
this is your chance to rise above that. To achieve despite it. I considered myself at a bad enough school (we've had duxes in the 80s), but another user round here got 99.70 or something, at a school with a median SS of 26, and ranked like 350. There are countless stories of this, because ultimately you decide your own performance. In the end, it's more satisfying to say that you've done it, against the odds, despite your circumstances, than to say, 'My school, my tutor, my money achieved that score for me'.