What's your FPSC version?
Do you use mods?
Which scripts did you use for the zones and the light?
I tried it this way (FPSC 1.16/ no mods):
The light:

lightoff.fpi as main script

the light must be set as dynamic

the light should have a unique name..ie: mylight
first triggerzone:

plrwithinzoneifused.fpi as main script

name of the light (mylight) in the IF USED field of the zone.
second triggerzone:

name of the light (mylight) in the IF USED field of the zone.

and a altered plrwithinzoneifused.fpi which sets activated to 0 to switch the light off:
;Artificial Intelligence Script
;Header
desc = Plr In Zone, activate entity specified in IF USED (sets activated to 0)
;Triggers
:state=0,plrwithinzone=1:activateifused=0,sound=$0,state=1
:state=1,plrwithinzone=0:state=0
Works fine.
The first triggerzone turns the light on and the second triggerzone turns the light off.
Hope this helps