The incident happened on the Oldpark Road in north Belfast just before 4pm on Wednesday.
Pupils from the Girls Model School were said to be terrified as their double-decker school bus, which was travelling to the Greater Shankill area, was boarded by a group of youths.
The DUP condemned those behind the “disgraceful sectarian attack” which they said had caused particular distress to first year pupils caught up in the ordeal.
It is understood the screaming pupils rushed to the front of the bus as they came under attack.
While the DUP claimed it was a group of youths, yesterday police appealed for anyone who may have witnessed the incident and reported it was a single man who boarded the bus.
They said: “It was reported that a man, described as being about 18 years old, around 5ft 5in tall wearing a blue top and with his face covered with a scarf, boarded a bus in the area and threw eggs at the schoolgirls onboard.”
North Belfast councillor Gareth McKee said: “The attack was carried out by five or six older teenagers described as being around 18-years-old.
“Some got onto the bus through the rear fire exit and threw eggs at pupils on the bottom deck causing pupils to rush screaming to the front of the bus, with [a pregnant girl] among those caught up in the crush.
“The nationalist attackers shouted sectarian abuse at the Protestant schoolgirls as they carried out the attack.
“Some girls were repeatedly hit with eggs.
“First year pupils were particularly terrified by this horrendous ordeal.
He continued: “The attackers took a folder of school work off one pupil and ran off with this.
“Another pupil had to get off the bus to retrieve this and she was pelted with more eggs.
“Some pupils think that their uniforms have been ruined.”
Councillor McKee appealed for anyone with information to come forward.
He said: “These cowardly bully-boys must be brought to justice. We appeal for calm and for no escalation.
“We will be maintaining contact with the senior PSNI officers, with Translink and with School management over the coming days as we seek to maintain calm in the aftermath of this appaling incident.”
Police have appealed for anyone who may have witnessed the incident to contact them on 0845 600 8000.