You should only be "discouraged," particularly to the point of wanting to quit, if you think you have no way out of ever doing anything other than the high school sports you're complaining about. And that is unlikely.
Let's say you actually do think you're a loser for covering high school sports all the time. What can you do?
Well, you could do what Doc Holliday suggested (or, the reverse, actually) and seek out some college-level events/features to write about, and start building up strong, varied clips. Even if you have to do it on a freelance basis for someone else, it could be done. Look up AP and seek out college game-coverage opportunities, look for sports stats/recruiting web sites, or specialty sports magazines (I did this quite a bit, and learned to love most of the so-called "minor" sports, and I became very knowledgeable about them). If there are any colleges/universities nearby, contact papers and websites of opposing teams that might be seeking coverage and find out about possibilities of game coverage or features about them.
Or, seek out different types of preps clips, even, that will, even to you, seem better, stronger and show some range. Trust me, you will feel the difference in the writing/reporting, and hopefully, in the finished product, compared to the countless everyday-basis, garden-variety gamers, and will know that you've added to your experience, and probably, your clips by doing that assignment. Even if it is still about preps, it won't feel to you like it is just about preps.
Look for hard-news sports stories involving players, teams, schools, etc., and project/enterprise pieces with broader range, appeal and inclusion in terms of voices, schools, stats, etc. Finding and recognizing these, and reporting on them, actually can be more difficult with regard to preps than with colleges/pros, and doing so will, again, show something good about you and your work. And you, again, will realize the difference. If you want an audience beyond just preps, sometimes you have to write about topical things that are of wider interest, even if they involve just high school athletes. Make the topic, not just the player, your subject. These also are the types of things future hiring editors will be looking for.
If doing preps at a large metro rather than the Podunk Press might make a difference to you, see if you can do some work for that paper, even if it's on a freelance basis, or involves inside phones/agate/roundup work that might eventually lead to more. Just a word of warning, though: If you go that route, do so realizing that it might not ever turn into more. But you'd be working at a big, recognizable paper, and for some people, that matters. Even then, though, you might not want to freelance forever, either. Just be aware of the pitfalls.
Most of all, keep seeking and applying for jobs beyond preps. But the point of all this is, you have to prepare for doing that, first by developing chops, and, often just as important, connections. Then, you can show someone who is hiring that you have them.