"I was furious, I spent time with the school and it was accepted - and I accepted him going back to rugby when he got back. Sam has already chosen to go the worlds and it was his choice," Mr Edwards said.
He accused the school of putting rugby over other sports.
"We had an agreement. I spoke to the previous headmaster and we discussed what would happen if Sam did make it [to the worlds] because there would be a clash.
"And it was agreed between us and Sam's dad if he made the world champs he wouldn't be called upon for rugby until the day he got back, and I didn't have a problem with that."
Mr Edwards said Sam's training this week would have to be changed, "because he's going to have sore muscles. He's not rugby fit."
He has trained Sam for four-and-a-half years and said he was committed.
"You can't not be committed at that level, you can't just be a freak. He trains two hours in the gym, four days a week ... These are specific exercises for throwing, not rugby."
Auckland Grammar director of sport Grant Hansen said it was the decision of Sam and his parents to play, and he denied any pressure had been put on them.
He said the school representatives had a discussion with them in March and it was agreed he would do both sports if that was what he wanted.
He hadn't been involved in a pre-season trip to Australia because it clashed with his discus training.
"He's an outstanding young athlete and we are fully supportive of high-performance sport, but there was definitely no pressure for him to play," Mr Hansen said.
Sam's father, Samiuela Ulufonua Snr, says he plans to speak to Auckland Grammar because he doesn't want his son to get injured.
"I didn't want him to go play rugby but they said they had no one else so I felt like I had no choice.
"I will go and talk to them. I don't want him to play until he [gets] back from the world champs."
