Dear Sick of CA,
I'm so sorry to hear of your frustration! Most of us have felt the way you do so we recognize your pain.
Since you specifically mentioned a city of 20,000 to 30,000 population, I searched around and came up with the following list.

How appropriate that it is from city-data, huh?
Missouri Bigger Cities (over 6000 residents)
Funny thing about the list, though. I was surprised to learn of the cities that are in that size range you requested. Turns out, I wasn't familiar with most of them! I have to comment that I find population can be deceiving, though. Other things factor in to make a difference. Proximity to other towns can be a huge factor! For instance, I used to live in Crestline, CA, which had a population of about 10,000 to 11,000. With only two exceptions (McDonald's and an Ace Hardware/Radio Shack combo), there were no chain stores or restaurants and only one grocery store, which was small but had most things you would expect to find...and a few more.

All of the other businesses were locally owned. On the other hand, Ozark is the same size and has a Super Walmart, Wal Mart and tons of other businesses, restaurants, drugstores, 2 other grocery stores, three off ramps on the freeway, several gas stations, etc.
Up in Crestline, there was an elementary school in town but the Jr. & Sr. High students from several towns were bussed in to Lake Arrowhead High School, if I remember correctly. They've got several elementary schools as well as a separate junior high school and a HUGE high school in Ozark.
I guess an especially good question for you, as parents of a teenager, to ask of Realtors when you talk to them is, "What sort of activities are there for kids in town?" and, "How much of a problem do you have with kids and drugs?" My understanding is that when there's nothing else to do, drugs often become a problem.
Employment options in an area are important, too.

That might be obvious for parents but it is also important that young people can find jobs when they're ready.
You've probably already thought of these things but since they weren't here in the thread, I thought I'd add them.

I hope you are successful in finding a place that meets all of your needs!