Two men exposed themselves to pupils from several schools after apparently hacking into a "meet the author" Zoom session.
Year 7 children in Doncaster were invited to the call organised by the not-for-profit Reading Rampage project.
The intruders reportedly "committed lewd acts and made threats" towards pupils from six schools, police said.
Organisers said they aborted the broadcast as soon as possible and were supporting affected children and staff.
Reading Rampage, which allows pupils aged 11 to 13 to meet authors, said the arrangements for Tuesday's call were "consistent with 11 successful recent sessions with more than 30 schools".
"In a rapidly unfolding sequence of events, the author's presentation was sabotaged and our meeting controls over-ridden. We aborted the session as quickly as we could," she said.
The author and organisers were "absolutely devastated", she added, and were co-operating with police.
Jess Hughes, whose children attend The Laurel Academy in Mexborough, previously told the BBC her two daughters had been very upset at what they saw.
She said pupils clicked the link to the lesson to be confronted with men on different screens exposing themselves and saying crude things.
"My girls have been really shocked and pale and quiet. I was distraught, I've been in tears all day."
Delta Academies Trust, which runs the The Laurel Academy, said a "small number" of students had taken part in the lesson.
It has since emerged that other schools were invited, although it has not been established how many children were watching.
A spokesman for Zoom said the company "strongly condemns such behaviour" and would work closely with law enforcement.
He added that the video-conferencing app had been recently updated "to help hosts more easily access in-meeting security controls".
South Yorkshire Police said the force was working with Doncaster Council to establish the exact circumstances.