Once there is a good, capable administration, doing their job, caring about kids, with a good, fair discipline system in place, then it is the time to start educating.  You need these things in place first, because without them you cannot teach.  In bad districts with low test scores, you can attribute the lack of success to those two things.  It's a bit hard to believe, but there are entire schools with behaviors so bad that no academics happen.  In these places, the 'school' is really just a glorified daycare.  That's why I suggest tackling those things first.  Because once all that is set, then you can make your school work well for everyone in the district.  

Most districts break their schools into some form of the following levels: Pre-K, Elementary, Middle School, High School, Special Education, Adult Education.  This will be a slight overview of what each of those levels is.  

I don't know much about preschool, so I'm not going to touch on it much, other than it gets students ready for school.  

Elementary school - K-5th grade.  In these years, students learn the basics of reading, writing, and math.  This includes learning what those things are, how you do them, and then practicing.  Thrown in are a lot of interest-based things.  Children see and explore the world around them.  They learn what goes into society, and the fundamentals of academic life.  (This is also not my area of expertise).   

Middle school - 6th-8th grade.  These years begin the academic and social training of students.  Occurring as students are going through puberty and their bodies begin to change, as well as their minds, they learn how to behave in a group.  They also take the fundamentals they learned in elementary school, and apply them to the broader world.  They spend a lot of time learning how things work, and do this in a safe space.  The goal is development.

High school - 9th-12th grade.  These are the years that begin training young people for what they want out of life.  High School exposes young people to many different subjects of school, interest based electives, and offers many sports, clubs, and hobbies.  These are offered so that young people can see what they like and what they're good at, where they fit in, and who they are.  The goal is that at the end of high school, a student is ready to pursue their own interests in a capable, honorable way.  Upon graduation, they are also adults, and are expected to be productive members of society.  

Special Education - This is the extra support for students with any type of disability, from mild to severe.  Special Education is a process by which students come to terms with their disabilities, and find ways to cope with and overcome them, so they can be productive adults.  

Adult Education - 18+.  Adult education is an extra service that offers language training and GED classes for students that didn't fit the traditional mold.  It's essentially a cheap way to help people that fell through the cracks get back on track.  

Rather than full details of each, the above is an overview of the major goals of the various levels of education.  When we educators get into the field, we choose what area we'd like to be involved in.  So I, for instance, went the secondary route - which is older students.  

Based upon the grade level you'd like to teach, you choose your level and subject matter and are trained accordingly.