D.R.
Level with the child. Be frank and inform him that he will be unable to attend the school he loves if he cannot use the toilet. One more inappropriate pooping incident and he will leave preschool. Do it without making it a big deal.
He may not be ready. You do not give us his age. My sons were 3.5 years and 3.75 years old before they were done with daytime diapers.
Then keep him home until the new term in September and try again. Maybe his body will be more mature and he will be potty trained.