Help With Grading Homework

I want to illicit others help in grading homework. So I need to give some access to others to the admin parts of the site. 

How can I set it up so certain members only have access to editing the "Assignment" module, but cannot change other parts of the site - or have access to it.

I have seen the roles and am looking to modify one of the roles .. probably the "teacher" role .. but what functions do I need to enable?

It looks like I have to give them full access to all or most course editing functions..  

PLus there are s many options in the Role Admit edit window. 
Is the re a re way to start by "deselecting all" first, then just selecting the limited permissions I want them to have. 

By the way , I still want them to have access to what they have as students..  These are level 2 students who will be teacher / students to those who just passed through level 1

I hope this makes sense. 
thanks