The vast majority of English teachers in middle and high school teach MLA formatting for essays with good reason, as it is what students will most likely be required to use in their first year of college. It is the format used for English and the Humanities, after all.
But after having taught entry-level writing at several different colleges, I found students won't always use MLA. At one college, all freshman were required to use APA, as the school's majors were primarily in the social sciences. APA formatting is the preferred format for the social sciences, and for many students with majors and careers in these fields, MLA will be nothing but a distant memory from high school.
Teaching APA on the secondary level is useful for several reasons. The most important, perhaps, is that when taught in addition to MLA, it will prepare students for college writing. It is good for students to understand that there are different styles for different types of writing. It will increase their flexibility in the different rhetorical strategies.
Unlike MLA formatting, APA style includes headings and sections, which can lead to more thoughtful and intuitive organization. Assigning one of the two most common APA-style papers, literature review or experimental report, will give students excellent experience in these rhetorical modes.
APA also has many writing style rules, which can seem overwhelming on the secondary level when we are trying to teach the basics of good writing, but I've found most of these style rules are already the things we are encouraging our students to do, such as writing in active voice instead of passive voice. When I teach APA to beginning composition students, we focus on the formatting and save the style rules for more advanced classes.
There are plenty of APA guidelines online. My favorite is the Online Writing Lab at Purdue, but if you would like some resources on the basics meant for secondary students, I've got some for your here: