Garnet Bell, 46, sprayed flaming petrol into an assembly hall where 31 pupils were sitting A-level exams, seriously burning three of them.
He told Antrim Crown Court he had "intended to spray fire on them" but had not meant to kill or cause them serious injury.
Bell, from Belfast, denies eight charges - including attempted murder - related to the attack at Sullivan Upper School in Holywood, Co Down, on 17 June last year.
He said he felt remorse for the attack "because the young men were so terribly hurt".
Bell admitted carrying out the attack but could not explain his actions. "I have thought about this for a long time and I cannot conceive what this could possibly have been intended to achieve."
He recalled walking into the hall, lighting the flame-thrower, seeing flames flaring out of it, and then some people burning. "That constitutes everything I saw in the hall. That is the absolute truth," he said.
Ronald Appleton QC, for the prosecution, asked Bell if he knew what the effects of the flame would be on skin. "I wouldn't wish to discuss it," said the defendant. He later said the flame-thrower, a converted fire extinguisher, "turned out to have been rather more powerful than I believed it was" but denied it was designed to be used against people.
Bell said a map of the school found at his flat with writing on it was "the most half-baked plan I've ever heard of in my life. I would say this was some sort of cracker barrel plan to go to the school. I would say the person was profoundly confused because there are arrows all over the place".
Bell said that he held no grudge against the school except for the fact that they had not given him "appropriate career advice".
He told the court he had taken prescription drugs on the day of the attack and said he had received psychiatric treatment while a pupil at Sullivan.
Earlier Bell pleaded guilty to a separate arson incident on the day of the attack. He admitted burning the home of his brother, for whom he felt "intense loathing". He said he would have tried to kill his brother if he had been home at the time.
Bell told the court that since the attack he has "become aware of how irrational I was".
During three-and-a-half hours of testimony, the Lord Chief Justice for Northern Ireland, Sir Brian Hutton, repeatedly reminded Bell to answer questions directly.
The jury have been told that they have to decide upon Bell's intent in carrying out the flame-thrower attack.
During the prosecution case, the injured teenage pupils told the court how they fought to escape a fireball in the classroom and saw the skin on their hands melting. The three seriously injured pupils have needed skin grafts and will have to undergo further surgery, the court was told. Three other pupils were slightly injured.
The case continues.