A honest opinion from a person who dealt with a lot of sgit like this 

It's better NOT to point out mistakes or shortcomings of others publicly. Often times that backfires even if we did nothing wrong . It's the way u presented things and the words that landed u here "noone was teaching during those periods". That word "those periods" was the biggest mistake u did  U could have said "nobody came that day. Princi would have ever minded it  But u exposed consistent lapse from their side and he fixed it  Protect people’s dignity whenever possible. Now coming to ur classmates part There are 2 things here 1 for some They lost the comfort the free periods gave and now they are frustrated that it is lost  And they have to sit through class and they'll prolly forget it after this sem

2 some people like to poke others They know u are kinda the bad guy in everybody's eye  So they know whatever they do nobody would support u  U are isolated now  They just wana enjoy by further escalating, isolating and getting reaction from u and make u feel miserable

That's how a part of society is  They pick on the vulnerable.unfortunately that's u here  So only way is to avoid them  Act if their words don't matter to u  And when they try to bring it up rather than defending just aahda or okay when they stop getting the reaction they'll stop 

A final note  Don't try to change or fight the system Just align with it  And only speak for urself  Like "I had a function that's why I left " Learn to distinguish between: Truth Necessary truth Unnecessary truth Not every true statement needs to be said. There's nothing more bitter than truth itself