I wouldn't be so sure about that, we've already seen Moonsong bend the rules to her liking several times. Let's say it comes to a "clash" between the principal (lower middle class government employee?) vs. Lady Steel & Moonsong (the poster child) and Parker (money and influence)... my money's not on the principal
This seems to me to be the real reason
Honestly, I agree that leaving him for MC is probably the biggest reason.
Narratively however, I agree the Sisterhood and Parker definitely have the resources to crush the Principal if they wanted. However, it's important to remember that he's not a meta (as far as we know) and the Sisterhood, being under the auspices of the MTF, are tasked mainly with dealing with meta threats and not "normies". So it might be trickier than normal to manage publicity around any action against him since it's not much a stretch that school rumors could reach parents and then broader news. It's not really a question of if they
could but if the effort of managing the media narrative would currently be worth it.
And on that note, he's obviously pretty sketchy but at the moment they didn't really have a direct reason to bring him down. The biggest personal motive would be because of him sending Cat to the farm, but we've just prevented that from even happening. And instead the Principal is sending people that (for once) probably actually deserve to be sent away. Strategically, so long as his actions are aligned with MC's goals (intentionally or not) it makes sense to continue to quietly gather dirt on him before moving against him. This is particularly true if some factions of MTF are willingly turning a blind eye or even supporting the Principal's abuse of power in sending students to the Farm.