Ben Megarry, who has autism, was charged with making a total of 23 hoax bomb threats between March and September 2012.
The 19-year-old, from Harmin Park Newtownabbey, pleaded guilty to 14 of 23 charges but denied a further nine.
The schools included Columbine High School, the scene of a massacre in 1999.
Each of the offences states that Mr Megarry "communicated certain information which he knew to be false ... with the intention of inducing a false belief that a bomb or other thing was liable to explode or ignite."
Appearing at Belfast Crown Court, he admitted making two hoax bomb threats to Columbine High School in Colorado on 16 and 18 April 2012, as well as Malibu High School and Chico High School.
He also admitted making calls to LAX Airport in Los Angeles, both Kansas International and Kansas City Airports, the Royal Stadium in Kansas and a Walmart store in Missouri.
Mann's Chinese Theatre
He then denied making a third hoax bomb threat to Columbine High School on 20 April 2012, and of making a further eight hoax bomb threats to other schools in Texas and California, as well as JFK International Airport in New York and Mann's Chinese Theatre in Los Angeles.
Mr Megarry's lawyer spoke of the "complexity of the case", and asked that it be reviewed again later in the month.
The judge agreed to this request, before releasing Mr Megarry on continuing bail.
A County Antrim teenager has admitted making some hoax bomb threats to US schools, airports and organisations.