Gary Dreher was left with severe cuts and bruises after he was attacked at the gates of St James' high in Exeter by teenagers last September.
A 17-year-old received 10 months in prison for the assault which occurred during an event when prospective pupils and parents were visiting the school.
Mr Dreher, 40, said he did not believe that longer prison sentences would have put off his attackers.
"I don't think that prison sentences are an effective deterrent," he said.
"Far better to spend the money on earlier intervention, because by the time young people like that are teenagers it is too late."
Mr Dreher said that smaller class sizes and greater investment in schemes that helped disadvantaged children, such as Sure Start, would make more sense.
"My attackers had a difficult background, but that doesn't excuse what they did," he said.
"They were out-and-out thugs. Even if they had put the sentences up to 50 years I don't think it would have deterred them."
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