French police in Thonon were placed on high alert after messages were posted on Snapchat suggesting the Jean-Jacques Rousseau School was to be attacked with bombs and threatening the principal with kidnapping.
The messages urged students not to attend school.
A 12-year-old boy has been arrested for sending the messages – he told police he meant them as a joke.
No evidence of an attack has been found. The boy will be in juvenile court next month.