Gary Dreher was off work for more than a week after he was assaulted at the gates of St James' high, a secondary school in Exeter, during an event for prospective pupils.
The 40-year-old was left with serious cuts and bruises. He said: "I had a couple of stitches in my face and my eye is still sore. All schools and teachers are vulnerable and that's a worry."
A 17-year-old youth has been charged with assault.
Devon county council paid up to pound;100 for four guards from a local firm to improve security at another open evening last week. A spokesman said: "It was something we felt was important to do."
A recent survey of 2,500 teachers by the National Union of Teachers found nearly a third had been physically assaulted almost once a year. In Manchester, guards were put on school buses this summer following concern about rowdy pupils.
West Bridgford comprehensive in Nottinghamshire employed four security guards earlier this year after two youths entered the grounds and attacked a pupil.
Up to 200 schools are thought to have a police officer based on their grounds as part of the Government's Safer Schools Partnership.
St James' is being rebuilt as part of a pound;79 million private finance contract. Security is at the heart of the plans, with internal and external fences, CCTV, security lighting, a designated visitors' entrance and a separate barrier-controlled access road for deliveries.
The Department for Education and Skills say pound;120m has been spent on school security since Labour came to power in 1997.