I'd also like to include my speculation that Suo-fei probably made an agreement to stay in furin under the condition he only be there for a semester max. He must've felt suffocated or bored out of his wits in his hometown and wanted a change of scenery. But look who ended up getting attached to the point he allowed one of Bofurin's weakest members (Nirei) to punch him, just once, as a sign of his guilt and form of 'punishment' for letting his feelings get to him.
Suo-fei, if you're being held against your will under some obligation or promise, blink twice. If Red Chanpuru is that territorial to the point they're strict about not letting anyone leave the island for another place, it'd be an even worse outcome. Furin kids are NOT gonna be able to take down a whole island that has experienced adult fighters in it dawg
Jokes aside, what the hell are the furin kids gonna do??? Not only is the Red Chanpuru territorial like Bofurin, they've been at it for decades which amps their hostility to outsiders to the max.
Whether Suo-fei came to Bofurin as a spy or out of his own personal interest, now it's gotten him caught up in a tight spot. Technically, he's made an official resignation to cut off all ties from Bofurin, going so far as to revoke his student registry there too. But we all know Sakura & his friends are stubborn and will insist they at least get a clear explanation on why Suo-fei stuck with them for so long.
He could've left when things were at their worst during the war arc, yet he stayed and fought with them.
With the current situation, it seems they'll make a trip to Red Chanpuru's island (it'll likely be a select few too so as to avoid sparking conflict since they know how territorial that faction is). Most likely, they'll ask to meet Suo-fei and demand an explanation but it'll be seen as the furin kids being hostile, thus escalating into a full blown "fight us if you want to meet (insert person)" scenario.