Around three-quarters of the 2,581 parents who answered a survey on Netmums, the parenting website, said they thought it was a bad idea, and that it contradicted recent healthy eating drives. The same number said they would also prefer teachers to use other rewards.
Nutritionists and eating disorder specialists also warn about the dangers of associating food with good behaviour or academic achievement. Children who believe they only deserve treats when they have been good will feel guilty eating sweets when they do not feel they deserve them. Such guilt can lead to secret eating or disordered eating habits, nutritionists say.
Schools are already banned from having vending machines that sell sweets or fizzy drinks.
A spokesman for the School Food Trust said: "We are concerned about inconsistency of the healthy eating messages if a child is rewarded with sweets in the school environment. We encourage teachers to use healthier food or non-food items as rewards."
One mother claimed her son had been given "massive" bars of chocolate by his teacher, causing him to become "obsessed" with the food.
Other parents complained that giving sweets caused dental decay, exacerbated hyperactivity and reinforced the connection between sweet foods and good behaviour. One wrote: "There are more effective ways to get the best out of children. Bribery never works long term. It makes those who may miss out feel bad and sends a wrong message. They need to learn to make achievements for themselves."
But not all agreed. Another mum wrote: "Children enjoy sweets. I don't see why we should deny them a little treat. If you give your kids healthy food at home, then a sweetie from a teacher is hardly going to harm them."
About a fifth of those surveyed did not know if their children were being offered sweet rewards.
Cathy Court, director of food and nutrition at Netmums, said: "Giving out sweets as rewards seems to be an issue parents feel strongly about, but has not been brought to the attention of schools. Parents often feel they don't want to cause a fuss by complaining, but as so many parents are against it, headteachers and governors should be discussing this with their teachers."
But a teacher who answered the survey said: "I see the amount of rubbish parents send as snacks to school every day. Leave poor teachers alone."
Research, page 30.