The dogs will likely be fine. You should set up a routine for the pup so he knows when to expect you home. There is also the crate training option.
Will a yorkie poo be ok left alone while I go to school? He would be with another yorkie.
It depends if it is a puppy the adult dogs will put it in its place. If they puppy does not become submissive(let older dog win/be in charge) to the adult dog they my fight leading to problems or death later on. It is better if you keep them in separate rooms or in cages.