Start a new part (ipt) file.
Exit sketch.
Click Derived Component.
Select the assembly (iam) file.
+ for all parts you want combined.
This might give you some information for a rigid assembly derived as a single part. If you are interested in the FEA of a moving part within it's range of motion in the assembly check out Applications>Dynamic Simulation. You can assign Gravity and other forces and then output a series of time-steps for FEA analysis.
Have your teacher check with the Inventor reseller for your area and perhaps he can get you the free Autodesk Official Training Curriculum for Dynamic Simulation and FEA pdf file for Inventor 11. There were changes to the interface for r2008 but between that "book" and the tutorials in r2008 you might be able to learn some useful stuff. If your teacher has a budget you could order the new AOTC book from
http://estore.autodesk.com
This is fairly complex stuff - really impressive if you can demonstrate some results. If you run into trouble post what you have and you might be able to get some help.