Have two teenagers with iPhones.  I have logon to the icloud account the phone uses.  It has a "find iphone" feature that works when location is turned off on the phone.  It sends a remote command to the phone to turn it back on and report the phones location.  The assumption is the phones location is being checked by the owner.  I only use this when I the "Find Friends" reports location is not being shared with me.

Have caught each teen doing this ONCE.  Lost phone for a week.  Has not happened again.  Phones are like drugs to kids today.  The internet is like a fix.  If it is turned off it is almost like withdrawl.  No it is exactly like withdrawl. 

The other thing you need to rely on is your other kids.  We have had one who tried leaving his phone at a friends house and then heading out.  The friend didnt leave the phone at home.  The other kids were able to track the friend and we caught the teenager.  Punishment has resulted in that not happening again.  I think he is still trying to figure out how we caught him.  You don't lie to mom and dad!