Yes, I, too, think that Hogwarts is the one thing Tom's more attached to than anything else. It's home for him, and I think that he plans to reside there if he ever took over the wizarding world. Actually, that is the reason why I believe he would have tried to make Hogwarts a Horcrux.
The death is a problem, but not only as far as Hogwarts is concerned, I also have a problem figuring out when the diary was made. The memory-Tom said he'd been preserved in the diary for fifty years, right? The exact copy of Voldemort's sixteen-year-old self. So far, so good, BUT: Which death did he use? When he killed his father and grandparents, he didn't have enough information about Horcruxes yet. He had already killed them when he asked Slughorn, and while he seemed well aware what they are when he asked, he neither knew how to make them, nor if it was possible to realize his goal of a seven-part soul. After all, that was the information he wanted, so it seems unlikely that he'd already made a Horcrux before. Then there was Myrtle. She died rather by accident, popping out of her cubicle when Tom set the basilisk free. It wasn't planned, and technically, Tom didn't kill her himself. Also, he turned the diary into a Horcrux to one day 'finish Slytherin's noble work', as he puts it, and prove Lord Voldemort was indeed the heir of Slytherin. Now, it was only Myrtle's death, almost resulting in Hogwarts being closed, that made him give up his plan and decide to write the diary in the first place. You know, no death, no closure, no need to have a diary finish his work for him. So, he couldn't have used her death either. Then, in Hokey's memory, Tom Riddle looks slightly changed; thinner, paler, more handsom than ever
, and an occasional red gleam in his eyes. His already changed appearance suggests he'd already made a Horcrux back then, but certainly not many. One, I'd say, not more than two. But how? DD thinks he hasn't killed in between, but he isn't sure. Why put emphasis on it, anyway, if it isn't important? I think he did in fact kill during that time, the question is whom. Mrs Cole is high on my list...
Anyway, if he turned Hogwarts into a Horcrux, and I'm really pretty sure he did, I think he would have done it after DD refused him the DADA job. I job 'they both knew Voldemort didn't want', and furthermore, a jod DD would never give him, and didn't believe Voldemort to expect to be given. DD then asks what it is that Voldemort really wants, but naturally, he doesn't answer. Maybe that's what he came back for, to turn Hogwarts (or the DADA position, I'm not quite willing to give up on that, yet) into a Horcrux.
Whoo, that was lengthy, sorry!