With regard to RIPA, as I understand it you do not need written permission to monitor anyone's email or files. You just need to ensure they are aware of the policy that states "they will be monitored for policy compliance"
Under the Regulation of Investigatory Powers Act 2000 (RIPA) interception is allowed for certain listed purposes without the consent of employees. The government's Interception of Communications Regulations 2000 authorise employers to monitor staff e-mails, Internet use and even phone calls at work, provided employers have a 'lawful' business purpose and that they make 'all reasonable efforts' to inform users of the monitoring.
... I assume it's got to be the same for pupils??
RIPA starts to conflict with the Human Rights Act with regard to privacy issues, but the general advice given by specialists is that you write your policy in such a way that states you can and will monitor any data on your network to ensure policy compliance, and to aid in resolving networking issues.
We do not ask staff to sign the policy and Personnel/Legal have ensured that we are within the law to monitor any email when we are investigating abuse of Council policy cases.
It would be worth asking advice of the legal section when you have completed your policy. If there are any cases where you receive complaints, then I guess you would ask the Legal department to act on your behalf."