1 site need to give users control over sub areas only

Hi

I have inherited a school site – what I need to be able to do is split the site down into year groups e.g. yr7, yr8 etc that is simple enough. However, what I have not found is a way to make the students who are responsible for adding pages, content etc only able to do that to their particular area e.g. yr7s only add, edit, delete etc the yr7 area/pages (or set of pages).

I suspect I need a plugin to do this – any suggestions – please bear in mind not very experienced with wordpress – so simple is good.

Rgds

Derek