What have you done already to find a school? Just written to them? Most schools only take on GTPs they have a prior relationship with, they like to know what they're getting and have a good idea that you'll fit in with the school/department etc. For this reason they tend to take on their former TAs, technicians, secretaries, volunteers etc. If you could volunteer in secondary schools then you would be in a much stronger position to gain a GTP placement.