I understand the people loosing patience with the MC, I do...
But take into consideration for a moment a few things.
He is not simply up against a super powerful school and/or country. There are literal gods in their world who control the school and send people to entirely different worlds giving them powers to fight against demons lords that threaten those alternative worlds.
Granted he is inarguably more powerful than these gods. It really cannot be argued because the first chapter(s) explicitly stated that the demon lord who summoned him, his cat, was sealed within her world because she was, quite literally, too powerful for the gods to deal with using summoned heros or directly themselves... hence the sealing.
Therein lies the problem. The gods basically control everything. Everything from granting power, monitoring power, to transfer of people from the "main" world to others and back. They absolutely will not respond kindly to that demon lord being out of her "cage".
The MC, who was summoned by the demon lord for her to train to kill her, so she could finally die (after "gaining" her "sanity" and effectively deciding to die but being unable to end it herself), decided to not do the one thing she summoned him to do, but instead show her "the world".
Could he fight off all the heroes and goddesses? Probably, maybe some sacrifice to do it on his behalf (quantity over quality, etc), and maybe not... But this does not matter.
If he is revealed it isn't like he just makes an enemy out of some super powerful school... or some country... nor would he just be making an enemy of all the goddesses. No, he would be making an enemy out of absolutely every world that demon lords appear in and the goddesses control (presumably all of them).
They either could not rest due to constantly being hunted, or worse, could not be part of civilization. Anyone who discovered who they were would utterly hate and fear them. They would have to live away from people completely (I mean, honestly, why wouldn't the goddesses spread their name and appearances to all civilizations they control if the MC made himself known and faced them?)
At that point the very reason to leave her "cage" becomes pointless. They may as well have just lived there indefinitely... And that wouldn't make much of an interesting manga after a quite short period of time... would it?
And, to anyone who actually managed to make it this far... congratulations, and sorry. I tried to shorten the above as much as I could...