He said he did so because the boy, a relative, had sworn at his wife, who is the school's deputy principal.
The boy denied that.
The tribunal decision says the assault followed a "history of issues" with the pupil, who had a record of poor behaviour at school.
Mr Allen met with the boy's mother to apologise and she told him of her forgiveness, but she later complained to police and the Teachers' Council.
Mr Allen also sought counselling after the assault.
"This is a case which has troubled the tribunal. On the one hand, for any teacher to assault a student in the way that [Mr Allen] in this case did is a series matter," tribunal chairman Kenneth Johnston said.
"On the other hand, the evidence is compelling that this action on [Mr Allen's] behalf was out of character, with references... speaking of his positive contribution to eduction in the area and his kindly approach to students and their families over many years."
The tribunal said the process was a "salutary lesson" for Mr Allen and there was no further risk to the boy and other pupils.
Mr Allen applied for his name to be kept secret to protect his children, who were said to have been subjected to "hurtful innuendo from third parties about the nature of their father's behaviour".
But the tribunal said Mr Allen's name wasn't suppressed at his court appearance and publication of the facts would benefit his family by avoiding suggestions there was more to it.
Mr Allen was ordered to pay 25 per cent of the complainant's costs, but was given a six-month grace period until these were imposed.
The tribunal heard his financial position was "far from strong".
"Although he did not provide us with details we accept that his resources are largely committed to the running of the school, and that he has very little, if any, disposable income," Mr Johnston said.