Michael Allen, 18, is charged with terrorizing and violating the conditions of release following an incident in February.
Officials said he was on an online gaming site when he talked about going on a shooting rampage at the school.
The FBI learned of the threat and contacted local authorities, who made the arrest.
In return for his guilty plea, Allen received a two-year deferred disposition, which means he can remain free as long as he stays out of trouble.
A judge warned Allen that he could have to serve a full five-year sentence if he breaks any other laws.