A recent ruling by the European Court of Human Rights makes it clear that individuals have the right to be told if their electronic communications are being monitored. Whilst the case concerned an employee, the rulings assertion that "spying is an intrusion of privacy", and that surveillance without the person knowing "amounted to an interference with [her] right to a private life".
The ruling means that employers, (and one would assume this applies equally in schools), must make it clear to individuals that they may be monitored, and that there is a very good reason for doing so. It cannot be clandestine.
Clearly schools have a duty to protect their networks and children from a range of threats, and there are now some excellent forensic software products to monitor misuse. What seems clear is that Acceptable Use Policies must let staff and children know that it is in place, and why. One for the e-Safety group to ponder methinks!