If your parents are the one footing your college bill, you might not have any other choice other than to go right after high school. I knew I needed to take a year off between high school and college, but I was paying for my own post secondary education so there wasn't really much my parents could say about that. Maybe you could come to a compromise with your parents if you are dead set on doing your working student position immediately after high school. Why is it that you don't want to wait? If you feel like you wouldn't be able to focus or commit 100% to college because you are burned out on the formal education system (which was why I waited), maybe you could apply to the college of your choice, and then defer classes for a year. That might show your parents that you are serious about college, and are still taking steps towards it, and won't just run off into the horse world.
As for finding a WS position, get out there, research on the internet, post on forums like this one and others, talk to eventing people, ask who they've ridden with, get recommendations from everyone. I've had a few friends do working student placements at eventing barns, all 3 really enjoyed their time, one worked for Kelli Temple, one for Kristin Schmolze, and another for CEO Eventing.
Some places will require you to come (fly or drive) out for a trial of sorts, you go out, have an interview, see the place and work/ride a few days to see if it's a good fit for both sides. That's one expense many do not consider or factor in ahead of time.
You have to consider what YOU want to get out of it too. There's all different types of working students, from grooms to riders to just strictly barn help and everything in between. Working students work long, hard hours, for little to no pay. You might get one day a week off, if you're lucky. You start right at the bottom doing the grunt work, then work your way up. Those who work hard, are ambitious and want to learn, do things quick, well and before they're told to and without complaint are the ones that get offered the "extras", like hacking the nice horse, that extra lesson spot, etc. You might not be riding horses right off the bat, and at some places you might not ride much at all.
If you have something specific in mind that you are looking for, make sure you're up front about it, like if you want a primarily riding job, or if there will be lessons in exchange for work and how many. Will there be any showing involved? These are all things you have to think about beforehand and be up front and honest about what you are looking for when first contacting places, so you don't waste their time and yours.
Including a short but sweet video with some clips highlighting some of your riding is never a bad idea. Make sure you and your horse are turned out to the nines though, and really showcase what you can do. Play up your strengths as well. Sure you might not have much show experience at higher levels, but it sounds like you have lots of experience bringing young horses along and taking them out to their first show which is a great skill, so I would emphasize what you are good at, and then include what you would like to learn as well, whether it be getting some show miles at a higher level on a more experienced horse, or if it's just getting out and starting eventing, etc. Don't make it about what you know and don't know, make it about what you can do and what you want to learn. Really sell yourself on your resume and in your interview. This is where being open and ambitious and excited to learn new things will work in your favor. Someone a little less experienced but who works super hard and watches and learns and is game for anything is going to get a lot farther than that more experienced rider who just wants to ride and thinks they're above the grunt work, or slacks off or has a bad attitude. Attitude is everything, and you really will get out of a working student position what you put in!
That turned into a bit of a novel, haha. Good luck