A police statement said the 19-year-old was arrested on Thursday at his home in the southern Swiss city on suspicion of planning a massacre.
According to news site 20 minuti/tio.ch around 20 pistols and other firearms were found on the premises, along with plans for the attack.
The suspect had in recent days threatened to kill an unspecified number of people at the high school, named as the Bellinzona cantonal business school SCC.
The broadcaster RSI reported that the teenager was suffering from a severe mental illness and had denied during questioning that he had intended to kill anyone.
Since his arrest he had been held at a psychiatric clinic it said.
The newspaper Corriere del Ticino reported that the teen was known to have a big interest in high-calibre weapons.
His Instagram profile picture showed him posing with a Kalashnikov.
Earlier 20 Minuti/tio.ch said students at the school were in shock on Friday and were being offered psychological support.
A mother with a son at the school, told The Local the situation there was being handled "remarkably well" by the headmaster and staff.
"There is a huge sense of gratitude for what was avoided," she said.
The mother said there had been a debriefing and a discreet police presence all day.
"The identity of the boy is no longer unknown, which of course makes it even more difficult to deal with/comprehend by the pupils," she said.
Teachers alerted the police to the danger after picking up warning signs.
“The school correctly interpreted the signs and identified that this young man, who is a brilliant student, had personal problems,” 20 Minuti/tio.ch quoted Ticino education department director Manuele Bertoli as saying.
Police said the arrest was instigated by a unit set up last year to prevent large-scale attacks.