Well, I helped my son do a clay brain for one of his projects (maybe bigger than what you need), and it wasn't
easy. It certainly wasn't easy to figure out how to do it well, and accurately, and also wasn't easy to make it.
His ended up with the brain stem parts removable as a unit, but no other parts were removable... it was one hemisphere only btw, with the cross-section showing on one side. Here's a photo of that hemisphere from the outside:
In this next photo, the hemisphere was lying down (with the cross-section on the table... oh, and the removable brain stem wasn't in place for the shot, and the cerebellum hadn't been added to the cortex yet)OOPS... just reread your message and see that only the lobes needed to be removable. That would be considerably easier! In that case you could make the cross-section actually flat (like the inner ear described below)... bake that, then add 3-dimensional lobes for the one hemisphere (maybe with scrunched aluminum foil armatures inside each to keep them from being thick enough to crack while baking) to the back side of the flat cross-section...separate each lobe from the other and from the cross-section with a tissue or a bit of ArmorAll to keep them from sticking together while baking. Guess you could hold them together after completion with velcro or wall-poster putty or something, which would allow them to be easily separated . .
So if you can
avoid the removable-parts option, I would definitely do that! (if you decide on it anyway, I can give you some things to think about and some suggestions).
You didn't say your age or the level of the class, so that could make a difference in how hard it would be re getting things more or less exact, or having fewer parts.
For an ear
though, I'd probably just pull up a diagram of one that showed the necessary partshttp://images.google.com/images?sourceid=navclient&ie=UTF-8&rls=GGLC,GGLC:1969-53,GGLC:en&q=ear
and go by that.
The easiest way would be to do it like a "puzzle-piece" painting or bas relief.
For that you'd make a flat base of clay (maybe skin colored) in the shape of the final model you want. Then you'd make each anatomical part in a different clay color, and press each firmly onto the base clay.
It would be helpful to lightly scribe a pattern on the clay to follow for placement... or you could do at least a pale "transfer" to the clay by firmly pressing a photocopy to the clay and leaving it for awhile before removing (remember to reverse image first, if you want it the original orientation on the clay) ...or you could just put the photocopy or drawing under a sheet of glass onto which you place the parts directly --no base clay--then bake on the clay and remove after cooling. For that last method, make sure all the parts are well connected to each other so they'll stay together when removed or handled.
After you do the basic puzzle-piece illustration and bake it probably, you could add clay to its back side if you wanted and baked again to give it some sort of 3-D shape.
And/or you could make the middle ear just a bit thicker by adding a thick sheet of clay, then also make yourself an outer ear more like a sculpt to add to it, as in many of the ear illustrations.
There's more info on the basic puzzle-piece "painting" technique on this page, if you want to look at it:http://www.glassattic.com/polymer/paints.htm
(... click on Bas Relief
, under "Polymer Paintings"...)
(you'd be making your pieces thicker, and even slightly bas relief perhaps, but the basic technique would be the same)