They feel that schools expect them not to "interfere" in the personal and social development of their youngsters, and that they are not told of any problems until they reach crisis level.
Some parents also said they had not met or spoken to their children's teachers for months or even years, despite attempts to inquire about the children's education.
More than 100 parents - mostly with children at boarding schools - attended a conference in London last week to discuss the social pressures affecting young people so they can become more aware and involved in their children's lives.
The "Teenagers" conference examined issues such as drug addiction, teenage pregnancy, under-age sex and the psychology of adolescence.
Caroline Noortman, the conference organiser, whose two children are boarders, said: "Many people believe that as parents we have absolved ourselves of our responsibilities towards our children because we send them away to school, but that is not the case.
"The attitude of the schools still tends to be 'give me the child and we will give you the man' and the underlying message from teachers is not to interfere. However, with the best will in the world teachers are unable to keep an eye on every pupil so it is important for parents to be involved."
Ms Noortman said most private schools were doing a good job and that pupils were being exposed to academic experiences that they would not get in day schools.
But she added that parents were often kept in the dark about problems involving their children. One mother said she did not realise her son was taking drugs until the school told her he was to be expelled. In another, a kitchen worker was dealing in cannabis.
Ms Noortman said: "These are clear examples of where the schools are failing in their responsibilities."
A spokesman for the Independent Schools Information Service said the views of parents had not been reflected in surveys, which suggested that boarders' families had good relations with schools.
Paul High, chairman of the Boarding Schools Association, said that courses had been set up with Roehampton Institute for boarding school staff with pastoral roles.