At the time, family members said two students were bullying the boy when one of them punched Bailey in the face, breaking his nose and causing a concussion.
O'Neill was checked out by doctors at DuPont Hospital in Delaware and released, but a few days later, he started having the violent seizures.
Butz, the superintendent, said the district was cooperating with Darby Township police, who were investigating the incident.
No charges were ever filed, and officials never released details of the altercation, nor directly connected the seizures with the schoolyard incident. However, it is not clear whether the death could alter that.
The office of Delaware County District Attorney Jack Whelan is investigating the incident, which was recorded by surveillance cameras.
Emily Harris, a spokeswoman for Whelan's office, called it an "active, ongoing investigation.
She said an autopsy will be performed and more information could be released following that.
The office is still assessing the video of the incident. The video shows O'Neill and a second boy involved in the incident. A third boy seen on the video did not take part in a physical act, according to authorities.
Meanwhile, the district is bringing in counselors for students.
"During this time, our thoughts and prayers are with Bailey's family and friends," Butz said in his statement, which was issued on the district website.
O'Neill was also remembered this morning on the social networking site, Facebook, where a page was erected in his honor after the incident became widely publicized.
"First of all I want to say the battle isn't over we will remember Bailey always and we will continue this fight until Bullying is eradicated," said a post on the Prayers for Bailey O'Neill Facebook page, which is not run by the family.
"With the heaviest heart we want to inform you all that Bailey has gone to be with the Lord please, please, please continue to pray for his family and continue on this fight for us until no child ever has to face a situation like this again. God Bless You all thank you so much!"
A family member reached at home this morning declined to comment.