The label "novella" doesn't really exist in YA. I mean, yes, "technically" it's a novella, but there are just very short YA novels sometimes and they aren't really called that. Since YA caters to teens, and some teens do better with shorter books, they're a staple of the genre. Living Dead Girl by Elizabeth Scott is 27k. That's the only book I can think of that is that short, and I think it's important to note that Living Dead Girl wasn't Elizabeth Scott's debut novel. If agents see a 26k YA novel from an unknown writer, I think the conclusion they're going to come to is that the novel is drastically under-developed.