If you don't have a CofA isn’t your insurance null and void?
That's true, but the legal implications might depend on who was responsible for the lapse of the CofA.
I am not a lawyer but let's say that a renter (or the lawyer representing his estate) is able to convince a court that the renter could not have reasonably known that the CofA had lapsed. Then the owner/operator (the school?) will go down for the passenger liability.
In my renting days, no renter ever checked any of the aircraft paperwork.
This accident took place outside the UK, but in the UK there is no passenger liability unless pilot negligence is proved. In this case that would perhaps not be so hard...