The Belgian support group Info-Timidité has a manual Vers la confiance en soi. It has identified around 60 causes of shyness, the most common of which are mistakes made by parents and teachers.
If your child acts like a loner, is rejected by others, rarely talks, ruminates obsessively or appears to be having trouble studying, think about your own behaviour or ask what is going on at school. Shyness is a prison. The earlier you deal with it the better.