I feel like they're more like comrades than mere high school students that's why they feel so strongly. Plus, he spouted nonsense like he never saw them as friends and stuff like that. Beating up Nirei when everyone knew of him as the weakest combatant was like salt to the wound. Anyway, that's about it. If Suo just changed schools then they would have been shocked sure but he didn't need to go as far as hurting Nirei emotionally on top of physically. That's probably why Sakura's angry.
Well I mean Nirei forced him to do it, it's not fair to put the blame on Suo, just because he's stronger. Suo only came to say good bye (he didn't have, but wanted to). Nirei on his end couldn't accept it and demanded an explanation. Suo on the other hand didn't want to talk about it and so was forced to say mean things. Same with Suo "beating up" Nerei, Suo just wanted to walk away, but Nirei wanted to fight him and on top Suo was still holding back, otherwise Nirei would've been in the hospital for a long time. In the end they're all mad at Suo, because they're selfish and put their own expectations on him. No matter if they're classmates, comrades, family or something else, when someone decides something for themself and doesn't want to reveal their reasons, the others should accept it, instead of being mad, trying to force explanations and trying to make them stay with fighting.

Just to clear Suo's name, he didn't betray anyones trust. Nirei wanted to fight him, because he didn't want to let him go and Nirei was well aware how a fight against Suo would end. Also Suo left on his own will, if it was due to his family or anything doesn't matter. He never made any promise to stay with them until graduation and has no obligation to give then a reason, why he left. And aside from the fact that they never have any lessons, they're still high school student switching schools is not so shocking. All in all most of the readers and the characters are just totally overreacting, because a high school student left his current school without telling his classmates beforehand. Aside from the fact that Suo said some things that sounded a bit mean, he didn't do anything wrong, so stop blaming and hating him, for no reason