Why didn't lily,james,cedric,dumbledore and everyone who was dead chose to act as a ghost,i mean,look at sir nicholas,bloody baron,fat friar or moaning myrtle!they can be ghost as well to stay at hogwarts!by why they didn't?

remember nick said it was the worst choice he ever made?
they dont want to live like that, bound to 1 place, feeling the pain of not being alive anymore
they would move on. Nick stayed because he was afraid- afraid of death. He regretted it though
it wasnt right for them.

You're asking a question that only JK Rowling herself can answer.

jk rowling said she was going to answer that but she never did, did she?

Remember when Harry asked Headless Nick if Sirius could come back as a ghost? Nick explained. I don't have enough time to type it all. I think it was in the 5th or 6th book, so read 'em.

It said in the end of the 5th book... READ IT!