A rumor can be countered using another rumor, they can point it out that who uses a real account (initial + birthday) to defame someone? They only need a little effort, but we can see that nobody is really doing something (except the one who asked the hacker, which is unrealistic in the first place)
If you really wanna do it realistically, the FL can even make a statement that she will find out the truth and go to an official government cybercrime unit to trace the account, this will allow the social platform to cooperate and find the culprit. Then the school has no other choice but to wait for the result because they will be legally liable for the decisions.
To add, she can also say that she will hold all those who spread the rumor reliable (This can make the rumor stop because they will be scared)
You can even put a bounty (prize money) to whoever can find the real culprit, or get evidence for those who spread the rumor. (This can also make others believe that the MC team is not the culprit because they are eager to fight for it).
The fact that they are just sitting ducks makes the rumor spread more.