According to authorities, the Sport & Medical Science Academy at 280 Huyshope Avenue was in a lockdown on Tuesday afternoon.
Hartford police said the threat of firearm violence came in just after 12 noon from a man using a computer-generated app.
Authorities said the caller said he would harm himself and others with a gun.
A SWAT team also responded to the scene and searched every room but found nothing.
The man who called in the threat has not been found.
Kids are expected to be back at school on Wednesday.
Additional security will be provided by Hartford police.