One of the blogs I regularly read is Confessions of a Community College Dean; a recent blog post in which he asked "How can high school teachers become college professors?" got me thinking, especially since I'm ensconced in teaching concurrent enrollment classes.
One of the biggest barriers I've seen - which is not to say that it's necessarily insurmountable, depending on experience and the facultyposition to which one aspires - is that many K-12 teachers have graduate degrees in education, as opposed to content matter. I think this can be a tremendous mistake. Teaching experience is gained by teaching - although I think taking education classes is immensely helpful as a pre-service teacher, and continuing professional development is key - but it's more difficult to gain that content mastery because of the amount of time one spends in a high school as a teacher; there simply isn't the same amount of flexibility as there is in the college world to travel to conferences, research, write, publish, or simply keep abreast of changing trends in one's subject matter.
If one wants to teach English, if one has an undergraduate degree in English but has a master's in education, the person hiring simply might not see how you have the advanced expertise necessary to teach English, even if you were a high school English teacher...unless you can show that you've taken some advanced courses in that subject matter.
This is what happened to me: I went to college later in life (I use the phrase loosely - I graduated with my B.A. in English when I was 31, in 2007), right when the economy collapsed on itself. I started but did not complete a graduate degree (which I have since done). Because of my extensive experience tutoring writing at multiple college writing centers, my mostly-complete master's degree in English with a concentration in rhetoric and the teaching of writing, and my formal teacher training, I found it exponentially easier to get a job adjuncting without the finished master's degree than I did finding a job teaching junior high or high school English, despite my being a licensed teacher. I adjuncted for a number of years in three different states, at both community colleges and universities (and in fact I was told I had gotten those CC jobs in part because I had attended and graduated from a community college myself; I would understand the culture from the students' perspectives).
In August 2015, eight years after having graduated with my B.A., I finally landed my first full time position teaching high school English; I was the first person to interview for this particular position, and was offered the job in part because of my college-teaching experience. My particular position requires me to teach concurrent enrollment (known in some circles as dual enrollment) through the same community college at which I've been adjuncting since 2012.
I did things backwards, in a sense, and certainly not the way most teachers seem to do it. I loved teaching at the community college and was about to just throw in the towel altogether - I was becoming cynical that I had put so much effort into trying to find a full-time teaching job in three states and having no luck.
I would in no way be able to recommend someone getting a graduate degree in education unless one is fairly certain that one wants to stay in the K-12 range, but this is just my perspective. I've appreciated the flexibility that comes with having a graduate degree in my subject matter.