Thats never going to happen getting all the schools to do it if your up front.
You could stop doing them and then say to a few folk that you think its funny that xxx school is burning engine hours for nothing. It will soon get back to them.
I can understand about wanting to keep instructors in the air but in general i think these vouchers do more harm than good to the industry. People are walking away from buying trial lessons purely because of bad publicity and that includes punters that would have taken your own vouchers.