The letter of warning was sent out across the city after the incident at a Leeds primary school.
The app in question was Live.Ly. But what is it?
The app is rated 'Parental Guidance' on the Google Play app store.
Advice for parents from website Commonsensemedia.org says: "Parents need to know that live.ly is live video-streaming in a social-networking app that allows users to broadcast live video to online friends and is made by the same developers as musical.ly - your music video community.
"As with any live-streaming service, there are concerns about what kids might see, what sort of information kids may give out, and the potential for bullying or inappropriate comments from viewers.