A George Town man "lost his cool" when he saw a man who he believed had bullied his son stuck up his finger at him, the Launceston Magistrates Court heard on Thursday.
Michael Allen Bishop, 51, pleaded guilty to one count of common assault.
Police prosecutor Tony Roughan said the complainant was driving to work along Anne street George Town when he drove past Mr Bishop.
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"The complainant looked in his rear vision mirror and saw Mr Bishop pointing at his car and yelling and he stuck up his finger," Mr Roughan said.
About 8.30am on February 20, 2020 Mr Bishop, who was accompanied by his wife, approached the complainant and grabbed him by the jacket.
"He said something about the school bus and punched him once to the left side of the face," Mr Roughan said.
As a result of the assault the complainant had a small split on his lip.
In an interview with police, Mr Bishop said he lost his cool after he saw the complainant stick his finger up.
Defence counsel James Kitto said the complainant had systematically bullied his son for a number of years.
He said his son had switched colleges.
"One day before an exam he was pelted with food and his son flunked the exam," Mr Kitto said.
"When he was given the bird he has gone to remonstrate with the complainant ... it is something that built up over a number of years."
He said the Bishop family home had been pelted with eggs by the complainant or associates.
Mr Kitto sought that Mr Bishop be sentenced without a conviction because a conviction would "torpedo his career".
The court heard that the complainant had admitted teasing Mr Bishop's son.
Ms Cure said that Mr Bishop had crossed a line and it was an assault that should not have occurred.
"There is an issue of general deterrence here, parents cannot intervene with violence," she said.
She fined Mr Bishop $1000 but did not convict.