William Forstchen, the author of the book One Second After, wrote what is basically a short story called Day of Wrath.
It deals with this type of scenario, Islamic Terrorists infiltrated the country, spread out to preplanned areas, specifically elementary schools in less populated areas.
On a set day and time, they attacked the schools, sealed them off and began executing children, while livestreaming it on the internet, when the authorities responded, other terrorists were in the surrounding areas to act like a blocking force to slow the response so more children could be killed.
The attacks were all over the country in a series of rolling attacks to keep everyone off balance.
Could a fictional story come true, something like that could easily happen.