[Moodle] classes or groups???

[lo at cgsc.vic.edu.au]

Hi everyone
I'm a new moodler and will be taking 3 parallel year 7 on-ground classes this semester with some online material and activities. I would like to have a student log into 'Year 7 IT' and then somehow end up just seeing material relevant to her class, like a class forum or particular worksheets. Also I would only be looking at one class's submissions at a time when I'm on-line marking.
Is this possible, or do I have to set up three different classes (and triplicating my work I think) each with largely the same worksheets etc but that way their forums and chats would be limited to the members of their own class?
I've looked at moodle.org etc but can't quite work out an answer!!
Many thanks
Jenny Looker
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lo at cgsc.vic.edu.au
http://www.cgsc.vic.edu.au/