are university students exempted form RT?
I thought they aren't exempted, even in-semester. Can try your luck with deferment, though. The decision lies with the unit CO, rather than anyone higher-up.
so can i book for my RT now? or wait till my unit send my a SAF100 for order?
Your unit will extend your RT window upon submitting the MC, to be commenced from whence your medical leave has expired. You can also opt to take the initiative with RT. Here's what MIW says:
"Q4 My family doctor had given me 4 months medical leave effective from my birthday. Would I be excuse from not booking RT?
A4 NO. It's the responsibility of the NSman to seek deferment from his RT obligations. You would have to submit your medical certificate to your NS Unit, who will extend your RT window. Your NS Unit will also book the next available RT schedule of your choice, to commence after your medical leave has expired. If you want to exercise your choice of RT schedule, you should advice your NS Unit of your preferred RT schedule and FCC when you submit your application for deferment.
Q5 My NS Unit has approved my application for deferment and had made a new RT schedule for me. Can I change my NS Unit's RT booking for me?
A5 You can only amend the FCC and the schedule options that your NS Unit had booked for you. You would not be able to cancel the booking as your RT had already been deferred to complete after 5 months of your current IPPT window. Any further postponement of your RT will result in the shortening of your current IPPT window, since you are only allowed to attempt your IPPT after you have completed your RT. If you want to postpone your RT to a later date, you would have to apply to your NS Unit for deferment again."
http://content.miw.com.sg/Mindef/Static/Ippt1/FAQs/rt_faq_index.asp