Subject to meeting basic rules laid down by the Home Office, each police force is free to set its own entry criteria. So the rules in your part of the country might be different from those elsewhere.
For further information, see here
http://careersadvice.direct.gov.uk/helpwithyou rcareer/jobprofiles/profiles/profile170/
and here
http://www.policecouldyou.co.uk/index.php
There are all sorts of rules and regulations which prevent schools from offering work experience placements with the police force. From the school's point of view, they wouldn't be allowed to offer such a placement, because it doesn't meet with the safety requirements which schools must observe. From the police's point of view, they can't (other than in very exceptional circumstances) have civilians working alongside them because of data protection issues (among many others).
Chris