Perhaps your student version imposes a limit on the number of nodes and/or elements you may have in any given model?
It seems to me that, at least for the purpose of demonstrating that this conical stress distribution can be obtained, you need not model the entire slab or all of the rebar. Perhaps you could model a relatively thin slice (narrow in the y direction) that includes only 2 or 4 rebar. Personally, I would strive for a conformal mesh between the individual rebars and the slab into which they extend so that no contact elements are created on the rebar/slab interfaces. You could also limit the extent of the geometry in the x direction, since stresses will likely be localized to the rebar/slab interfaces.
I don't know what kind of contact got created in your model in which the rebar was modeled with beam elements. I suspect there may be no connection between the beam elements and the slab under the surface of the slab, in which case the model would not adequately represent the physical connection. So, without employing other "tricks" that may require the use of APDL command objects to access MAPDL solver features that are not natively exposed in Mechanical, I would for now recommend sticking to modeling the rebar as solid cylindrical bodies.