Reading is tested informally by classroom teachers and more formally by periodic standardized tests administered statewide and nationally. Obviously, the goal is to have all students reading on grade level or higher. Since textbooks are written on grade level, students who have below grade-level reading skills will need support and specialized assistance if they are significantly below grade-level.
To gain more specific information about the reading levels in your area, contact the local school board. The results of composite standardized scores are available to the community and often the school district publishes the information annually. Each school also has test profiles about its composite student body.
In a middle school, reading is usually taught both as part of an interdisciplinary approach and "across the curriculum." As students encounter specialized vocabulary in each subject, and as the material varies in style of presentation, students usually receive reading instruction in numerous classes. Most frequently, reading instruction is emphasized in the language arts program for all students and supported by specialized reading instruction for students significantly below grade-level.
Check with your local district to learn more about how that school system is instructing students in reading. As a future middle-school teacher, you may find helpful information published in the journal or on the website of the National Middle School Association. Don't forget to do a search here at the FamilyEducation.com School Help section.