Because people just don't give a fuck about others. Most people will sympathize with you, but few will actually help. Using the example you're stating over there, in a class consists of 26 person, not all actually 25 person just want to run a living hell for that one person. the person who's really behind the curtain is usually just one person, or even nobody at all, starts by just a rumor made worse.
If you say a lie many times enough, it'd become a truth, at least from another person's eyes.
It's even worse in Japan where suicide is tolerable culture (at least long time ago). where people just hold everything inside.
(出る杭は打たれる) the nail that sticks out gets hammered proverb is a perfect words to describe it. what's different is the 'standard' line where the hammer stops hitting, which in Japan is simply lower than another country, i guess.
heard even worse in korea from a korean friend.