Gorka,
In your example you have defined a mixed shell and solid model. The solid extrusion is being meshed with solid tet elements, and you have applied a shell "coating" to the top surface of the solid. 
See attachment for preview of your mesh. Not sure if this is what you intended? If you want a shell only model you should either define shell pairs (Refine Model - Idealizations - Shell Pair...), or use a thin feature when you make the extrusion, or create a surface (not solid) feature. You have in essence made a solid block then added stiffness on one side. Hence it is no longer symmetric, and thus you will see deformation to one side (it bends like a banana!)
Also, your material is isotropic, whereas for composites you'll probably (definitely?) want to use a "transverse isotropic" that allows you to define properties in fiber direction. When you do this, the material orientation you define will be meaningful.
Finally, your layup definition has the same material and same orientation for all layers. You could have done this with a simple shell definition of 1mm. Granted you can now go back and add a thick foam core layer (perhaps in between plies 2 and 3).
I hope this helps? Please let me know if I have misunderstood your question.
Greg
