The list of suspects is indeed limited, but having that guy be the culprit would make Erased's case end almost like a Scooby Doo episode.
Satoru's mom knew who the murderer was after thinking about it for a bit, so that leaves out most far-fetched answers such as the guy who left his family after he was accused of shoplifting or alternate versions of Satoru. Yuuki was the overly conspicuous red herring suspect, Kayo's mom repented last episode, her lover had too little screentime to randomly show up as the murderer sometime later, and that means there's nobody left among the people that have been introduced in the show so far but Satoru's teacher, who just happened to conveniently had three very suspicious bits of evidence pointing towards him (candy in car, happened to be taking the same route that girl who got killed in the future usually took, and the stuff in the bus disappeared right after Satoru told him about where they had been keeping Kayo hidden).
While you could say that's just too convenient, the low remaining episode count means we're either approaching the end-game or getting a "wanna know who did it? read the manga!" ending, and there isn't anybody else I can think of that could be the real killer if the clues pointing out towards the teacher end up being red herrings as well.
Then there's the whole "Kenya talking with teacher" scene, but the killer didn't know Kayo was being kept inside the bus until after Satoru told the teacher, which also happened to be right when the killer conveniently decided to move all his/her tools of the trade out of there. It was even him who suggested having Kayo hide away in that bus, and had he been an accomplice the supposed murderer who visited the bus wouldn't have freaked out like he/she did that night Kayo was there.