Interesting dilemma. Not all that familiar with the Police SOP,
but I think you have to be prepared for the worst. Granted, the
local armed forces tend to "compromise" with local academic
institutions more. But, as a policy, reservist deferments generally
happen only for clashes-of-timing with examinations. (In other
words, you can get NSmen who paid $20,000 to do part-time Masters
but weren't excused for ICTs & ended up carrying textbooks
during field training!)
Keep an open mind, & be prepared to do a lot of legwork cuz
you're likely to deal with administrative folks (rather than anyone
with executive powers) of both the reservist & polytechnic
sides. Obtain a student-status letter &, more importantly, a
relevant academic timetable which you can submit as evidence. Then
write directly to your unit & apply for deferment ASAP. It's up
to the SPF people to access whether they can release you. You may
need to argue down the phone with a pencil-pusher, like this
frustrated SCDF fellow who ultimately complained to the Cut Waste
Panel:
http://app.mof.gov.sg/cutwaste/suggestionview.asp?id=326
If they insist you attend, then you'll have to apply for official
absence on the polytechnic side if I'm correct. Reservist is
different from national service in that there's no rigid sense of
"service exemption" associated with the former - well, except for
the exit permit that shows you'll be overseas. That said, it's the
unit commanders who actually have the final say.