Follow up on previous enquiries. If someone has given you a call in the past about learning to drive, call them up and find out if they are still interested in getting their full British licence (note I said 'getting their full licence') they might have rejected your services in the past because they wanted cheap driving lessons, but having gone down that route of 5 for £50 an hour and failed due to improper instruction, or found out that a lot of information had been held back from them, they might wise up and consider paying for high quality services from honest fully qualified instructors like you. I prefer to call rather than text, as it is more effective in converting enquiries to sales.
Be of help to learner drivers on online forums. If you show yourself to be knowledgeable and consistently helpful to provisional licence holders who are either being taught by another instructor or family and friends, then you might start seeing people wanting to use you either to pass the test after a failed attempt or transfer to you from family, in order to increase their chances of passing. Make sure you include a link to your website in your profile and signature that is appended to every post you make if allowed (see why you need a school website?). If you need more learners to take driving lessons with you, then you have the time to do this, and while the results might not be immediate, it will come if you persist and are consistent in being helpful to others.
Check up on lost pupils. Some of your old learner drivers might be thinking of returning to driving, and a call from you might be all they require to kick them into gear.
6th form college. If you have a college near you, then you might try and see if you can give a talk on topics such as 'Being self employed and running your own business'. This might generate some enquiries in addition to the opportunity of giving out cards to 17 and 18 year olds. I’ve gone at lunch time and handed out flyers to students with good results. Does a sixth form college near you have a magazine, if so find out if the take advertising and how much. (Always ask if other driving schools advertise). If you already take students from the college, do the students have a notice board in their
common room that you could get a flyer put up.
Put graphics on your driving school car. While having a roof sign is good, it doesn't advertise your services when you are not giving a lesson and don't have it on your vehicle. If you lease your car, then you could opt for magnetic marketing signs which can easily be peeled off.
When I started many years ago I approached the local Territorial Army (now Army Reserve) with the view of teaching the recruits or upgrading drivers to HGV (as it was then). With the government looking to have an Army Reserve of 30,000 some of these will need to be taught to drive.