People gathered outside the event centre were also playing loud music through a "megaphone".
Police confirmed they were notified just before 8 pm of a group of people in a car park reportedly blocking access to a function at Ellerslie Events Centre.
"Police attended and observed some people and motorbikes in nearby car parks but no entrances blocked and no offences being committed."
The witness understands the school ball is for students who attend Epsom Girls Grammar.
A similar incident happened earlier this month where students arriving at the Westlake Girls' school ball at Eden Park were overtaken by hoons with blaring music riding on car roofs and bonnets and hanging out of windows.