Though while it is probably more difficult it is probably very important
to structure different lessons for the different levels of Art.
Otherwise you could loose the incentive for students to see Art as a
sequential subject area, and they could get the wrong idea that only the
talented should take art, and / or I already did that... she does that
with all her classes, so why should I take Art again.
My advice would be, to build a sequential program. Group the students
according to the level they are enrolled. To make it easier you could
offer similar materials 'or' subject matter at the same time, and
stagger your introductions so it is do-able for you.
You certainly want to highly encourage those who possess great ability
in art and keep them coming back for all you have to offer them as an
instructor. Start a chapter of the National Art Honor Society and get
this group activated in applying and celebrating their abilities.