Peter Harvey, 50, was handed a two year community order on Monday at Nottingham Crown Court after he admitted bludgeoning the boy at All Saints' Roman Catholic School in Mansfield.
The "fundamentally decent man" was provoked by pupils during a lesson in July last year, when they called him a "psycho" and "bald-headed bastard".
He lost control and hit the boy about the head with the 3kg weight while shouting "die, die, die".
Following the sentencing, his union, the NASUWT, said it hoped the school's governors and Nottinghamshire County Council would allow the married father-of-two to retire on the grounds of ill health.
But today, the union said he was sacked.
In a statement, it said: "It is with deep regret that the NASUWT has to announce that the governors of All Saints RC School in Mansfield, Nottinghamshire, have today summarily dismissed Peter Harvey from his post at the school."
Chris Keates, the union's general secretary, said she was "shocked by the lack of compassion" shown to Harvey who had "dedicated the best part of his working life to the school".
She said the teacher of more than 20 years remained full of remorse for the attack, which left the boy with a fractured skull and limited hearing in his right ear.
Ms Keates added: "No one has ever sought to minimise the wrong that Mr Harvey did when he assaulted the pupil, least of all Mr Harvey.
"All those who have had any involvement in this case, including the jury and the judge, have, however, been prepared to recognise that here was an exemplary teacher, in a fragile mental state, brought to breaking point by a number of critical failures on the part of his employer.
"He has suffered imprisonment, endured a harrowing trial and been convicted. His career is in ruins and he has no income. His wife and two school age children, all with their own health problems, have had their world turned upside down.
"How much punishment should one person be expected to take? The summary dismissal meted out today is further punishment.
"His employers have not even allowed him to be dismissed with a period of notice and a modicum of dignity."
Ms Keates said the union was now taking legal advice about possible action.
A spokeswoman for Nottinghamshire County Council said: "We are not making any comment at this stage as there is a possibility of an appeal and we must not say anything that could prejudice that."
A keen singer and church-goer, the father of two teenage daughters, one of whom has Asperger's syndrome, Harvey was described as a "charismatic" teacher who cared deeply about his pupils.
But as he tried to support his wife Samantha, who herself has severe depression, he found himself driven to the edge by unruly pupils.
On one occasion, a boy who could not be disciplined by a colleague was placed in Harvey's class only to assault him, while another yob pushed him into a bush after he stuck up for a female teacher.
In December 2008, Harvey was signed off after telling the school's education adviser, Shahrukh Mugaseth, that he was having violent thoughts and feared he would seriously harm somebody.
But he was allowed to return in April last year after receiving therapy from a counsellor who told him he was too "gentle and placid" and needed to be better at letting his anger out.
Cruelly, pupils in Harvey's Year 9 science class took advantage of his health problems.
In the moments before the attack, one of the boy's classmates, a girl now aged 15, secretly filmed Harvey telling him to put down a wooden metre-long ruler he was using in a mock sword-fight with another pupil.
The taunting was part of a concerted effort to wind up Harvey so his reaction could be caught on camera and the footage passed around the school in order to "humiliate" the teacher.
The final insult, which lit the "blue touch paper", came when the boy told Harvey to "F*** off". The teacher responded: "I'll teach you to "f*** off", before, in actions described "like a man possessed", he grabbed the boy by the collar and dragged him out of the classroom.
CCTV footage released by Nottinghamshire Crown Prosecution Service following the trial shows the moments pupils crowded around a storeroom opposite Harvey's class as he hit the pupil twice about the head with a 3kg (6.6lb) dumbbell.
The clip, shown to jurors, shows the pupils then running for help as Harvey emerges from the room.
Meanwhile, the youngster, who cannot be named for legal reasons, is lying out of shot, critically injured.
Harvey always admitted attacking the boy but denied he intended to kill him or cause serious injury.
He went on trial for attempted murder and an alternative charge of grievous bodily harm with intent. If convicted he could have faced a life term behind bars.
But, from the evidence of witnesses during last month's trial, it became apparent that Harvey had lost control when he attacked the boy and could not have intended to harm him.
After the jury took less than an hour to reach its not guilty verdicts, Judge Michael Stokes QC, the Recorder of Nottingham, said "common sense had prevailed".
