Angry parents who criticise the school for their children's perceived under-performance are doing harm to their offspring, says Laurence Steinberg, author of Beyond the classroom: Why school reform has failed and what parents need to do (Simon & Schuster). His 10-year study of 20,000 US students in ninth through 12th grades - teens, essentially - found that parents play an important role in closing the gap between a child's ability and his or her actual achievement.

Ability alone does not determine a student's performance in school. While some researchers have noted a connection between homework and academic success, the study found that parents checking homework, monitoring academic involvement from home and encouraging better performance were not sufficient in themselves to raise students' levels of performance.

More valuable were the types of involvement that physically brought parents into the school - attending school programmes, parent conferences and extracurricular activities. One suggestion is that the reason the latter kinds of involvement influence academic performance is that they communicate important messages about the value of education.

Parents of successful students "work the system", mobilising the school on their child's behalf and communicating a powerful message to the child and school - a belief that the school can and wants to educate the child. This confidence, in turn, strengthens the child's belief in the school's effectiveness.