You have probably reviewed the local laws with your students, but you should definitely keep on reminding them as the laws in Singapore are some of the strictest in the world. Their culture is very different, and some things that would seem OK in the US can be considered crimes in Singapore.
No littering. No chewing or possession of gum is allowed, no smoking in any public areas, and definitely no drug consuming or dealing (these offenses can result in big fines and imprisonment). Tell your students not to bring any knives/weapons in their luggage as it is illegal to bring any weapons into the country without permission.
some information about local laws in Singapore:
http://www.smartraveller.gov.au/zw-c...ore#Local_Laws
http://www.expatsingapore.com/general/law.htm