No you can't give the improvement exam if
you have joined any course. The prerequisite
of giving the improvement exam is that the
concerned person must not be enrolled in
any course while applying for or giving the
exam

So, you can either join a course or you can
drop a year to give the CBSE improvement
exam the following year, whichever you are
comfortable with. Also if you lagged in just
one subject, there is an option which allows
you to give one exam the same year to
improve your score. This exam is usually
conducted by the CBSE along with those
appearing for compartment papers. But
again it depends on your choice to give the
exam or not as the results are announced late
around August when colleges seats are
occupied