Head lice are a common problem, particularly in school children aged 4-11.
They're largely harmless, but can live in the hair for a long time if not treated and can be irritating and frustrating to deal with. Please see the following website for details on how to deal effectively with occurrences of headlice and help prevent their spread:
http://www.nhs.uk/conditions/head-lice/pages/introduction.aspx
There is also a handy guide, covering the same points, that can be printed out at home: