Is it legal to take photos of others without permission.
In public, yes.
There is no reasonable expectation of privacy in public. Everyone can take photos in public places and you have no rights against being photographed in public.
If she were trespassing to take photos in a private location, or photographing people in a restroom, it'd be a different story.
That is why drones are becoming a sticky subject: A drone would be able to see over a fence while neither the drone nor the operator is trespassing. A person may have taken reasonable measures to protect their backyard from prying eyes by putting up a fence or a wall, but a drone renders their efforts useless. You don't really have privacy these days even on your own property unless you're inside the building with the curtains closed.
And use those photos to make controversial posts?
If you posted truthful information that was legally gathered and it happens to spill secrets that others didn't want shared, it's their own fault for failing to limit private activities to private places.
If you only share information that is true, you're safe from the law - even if you piss people off or ruin their lives.
(Though that doesn't necessarily stop your perceived
victims from trying to settle personal grudges.)
Now, if you use them to mislead people by adding slanderous spin - as
news agencies often do - that is how you get hit with a defamation lawsuit.
What's more concerning is this trope of it basically being normal for the delinquents to beat up the guy who gets the cute girl out of jealousy. Teachers and students look the other way. No one ever gets suspended or arrested for assault. Any guy who wants to date a cute girl has to tip-toe around the bullies because everyone is supportive of the assailants and the rule of law doesn't exist. The fact this happens so often in manga would imply that this is a real problem in Japan.