By the time most children get to high school they should be able to read so you have to concentrate on reading comprehension. There will be a few kids who have fallen through the cracks and you might have to spend some time with these students.
The best way if you can do it is a buddy system. This way you pair up a good reader with a struggling reader and hopefully the magic happens but not always. Some kids just don’t want to learn.
If some children are behind in reading and they want to learn bring out the Montessori sandpaper letters with phonics and see if they make a difference. If the child's parents are responsive to suggestion and don’t complain about spending money to help their children ask them to look at an online phonics program.
All high school students should have a dictionary and know how to use it. This will save you time so make sure the students get into the habit of looking up the words they do not understand. This will help the student’s reading in the content areas also.
To improve the reading ability of high school students the students must read. As they say practice makes perfect. As a teacher I have noticed that the students who read in their spare time in class are usually the better students.
If most students just got a lot more practice in reading they would be better readers, it’s as simple as that. But there are a lot of distractions in today’s world and there are better things to do, like playing computer games and watching TV.
I have found out of a class of primary school students only about 4 or 5 will carry a book around to read in their spare time. Not a good sign. Also the students who just read or look at a school book when they have a few spare minutes like the 10 minutes between class are always the better students.