The rules for selecting any topic for a school essay are always the same. If you have a passionate interest in a particular topic or a great deal of knowledge about that topic or, best of all, both of these things, then you are in the box seat as far as writing a successful essay is concerned. If you believe in the topic you have chosen or if it is something you would like to study in greater detail, then again you're well on the way to being successful.
So if you are given a school essay topic which is to deal with the police, then you should be looking for the type of topic which appeals to you. Remember that the topic of the police is extremely varied. It could include looking at the work done by the police, the police officers themselves, their training, career opportunities as well as the role of the police in the criminal justice system. To be truly successful in your writing you need to think of a topic or discover a topic regarding the police which has a particular interest for you.
Here is a selection of school essay topics on the police all of which can be found on the web. Read them through and see if one or more as a particular appeal for you. Maybe there is a topic here which you could tweak a little to make it even more appealing.