Liam Lyburd has admitted possessing a gun, bomb and bullets but said he had no intention of hurting anyone.
Newcastle Crown Court heard how he had been "thrown out" of Newcastle College two years earlier for being disruptive, but said he "liked the place."
Lyburd has denied eight charges of intending to endanger life.
During cross-examination, he said Facebook messages he posted about an imminent attack were a "wind-up" to try to maintain friends' online interest in him.
Nick Dry, prosecuting, asked if he had not been taking a "big risk" in buying a gun if it was just a game.
Lyburd replied: "Buying a gun was just like buying a bar of chocolate. I didn't see it as a big deal at the time."
He also told the jury he never thought anyone would know about his Glock semi-automatic pistol.
"I never thought I would get arrested, I thought I would keep it in my bedroom. No-one would ever know," he said.
He added: "I like Newcastle College, truthfully. It was sad. I wish I said a different college, I like the place."
Lyburd has pleaded guilty to nine charges relating to making five pipe bombs, two home-made explosive devices, possessing a 9mm Luger Calibre Glock gun, 94 jacketed hollow point expanding bullets and CS gas.
Asked why he needed so many rounds of ammunition, Lyburd said: "I was watching videos on YouTube and the Amercians, they have thousands. You can shoot 100 rounds in a few seconds."
Mr Dry asked if he would fire those rounds. "If someone kicked my door in I could defend myself," the teenager replied.
The trial continues.
A 19-year-old accused of plotting mass murder at a college said sourcing a pistol online was "just like buying a bar of chocolate", a court has heard.