This is a subject I`ve thought about over and over due to my personal background (ex LEO). Since an instructor can have his/her personal life brought into question involving an investigation of any diver he/she has certified, then YES there should be drug tests on instructors AND DMs.
I have heard several horror stories about dive leaders conducting classes or tours while on something (be it drugs or alcohol) which is inherently dangerous for all involved.
If I were on an investigating committee, that would be one of the very pertinent questions I would want answered.