You must ask yourself why you wish to drop out of JC. If it is
specifically the lifestyle as a JC student, you may still wish to
get an 'A' level cert even after leaving JC.
You may wish to consider doing private 'A' levels at the same
time, while doing your poly diploma. This is of course, best done
when your poly course and your 'A' level subjects are similar or
related. Eg. chemical engineering poly diploma, take H2 Chem, H2
Physics, H2 Maths at the same time.
I've tutored chemical engineering poly students. There's
a significant overlap with the H2 Chem syllabus. Both
will help prepare you for the other.