The teenager, from Plymouth, pleaded guilty to three offences, committed between October 2014 and January 2015, at Plymouth Youth Court.
The boy, who cannot be named due to his age, admitted targeting Florida's SeaWorld theme park and Devon and Cornwall Police in the attacks.
He was charged with the five offences following the investigation.
He has admitted three counts of doing an act to hinder access to a programme or data held in a computer.
He has denied two charges of sending bomb hoaxes to US airlines via Twitter.
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He said the tweet read: "One of those lovely Boeing airplanes has a tick, tick, ticking in it. Hurry gentlemen, the clock is ticking."
The tweet was also tagged to the White House Twitter account and the FBI were notified, Samples was quoted as saying by the BBC.
No action was taken by the US authorities following an assessment of the credibility of the threat, and the matter was passed to the UK authorities, the court heard.
A similar tweet saying was also sent to Delta Air Lines on the same day, Samples said.
The prosecution said the boy had changed his story about whether he sent the tweets during police interview, before finally denying the charges.
A twitter account used by the boy later tweeted the Zephyr Regional Cyber Crime Unit, saying "to be fair they caught me red handed" and "I still maintain the utmost respect for Zephyr", Samples said.