Regardless of how they're trained riding school horses are chosen for safety, willingness to tolerate rider mistakes, and being easy enough to take multiple riders. Once out of a school that can change, many a rider who's considered advanced in a riding school has come unstuck when they move to privately owned horses, particularly if they've taken on something that is suited to their perceived rather than actual skill. This is probably even more true where people have ridden at good schools since they're the ones who take particular care when selecting school horses and also in maintaining their schooling.
What I, and I expect
@newforest , are trying to say
@charlottlefish is that you've only had very controlled and safe experiences with horses. You haven't come across the sensitive ones, the sharp ones, the badly trained ones, the youngsters, or ones with problems. And if you decide to go ahead and work with horses you will be expected to deal with them and no amount of book learning is going to be enough because no horse has ever read a book. They can be useful to give you ideas on approaches to try but you have to have the experience to see what you're dealing with and stay safe in situations that can escalate quickly.
If you're absolutely certain you want to be a riding instructor I'd suggest this. Finish school and get the best qualifications you can. For a few years work in another area and use that time to get a lot of hands on practical riding and handling experience over a wide base not just riding schools. Then if you still feel it's what you want to do work through your BHS or ABRS stages to become a qualified instructor. You might also want to look at other careers that are available in the equine industry, there are a lot of options out there.