The boys allegedly planned to injure and kill several people with a knife and explosives, but the attack was not carried out before they were arrested.
The three boys, aged 16, 16 and 15, were arrested on Monday morning and are suspected of terrorist offences. The three used the messaging services Discord and Telegram to communicate with each other about the attack.
The detention hearings began on Monday afternoon and are being held behind closed doors.
Two of the boys were arrested in East Jutland and one in Copenhagen, according to police.
The police assess that there is no risk to the school in question.
