For a student who has difficulty keeping up with the rest of the class in terms of reading-level ability, you may need to work with the student on a one-on-one basis. Use the books the rest of the class is reading as a guide and have the student read excerpts from the book to you. Going through the material, either page by page or chapter by chapter, will help you learn which areas are most difficult for him. Knowing if the issue is rooted in comprehension ability or word recognition will then help you determine that best course of action.
A student who struggles with grasping the meaning of books may have a lower vocabulary knowledge base compared to other students. Using level appropriate books, create vocabulary sheets for her. Have her review the list of words and meanings and go through the list with you to further learn the words. Increasing vocabulary skills can be useful in getting reading levels up to par with peers.
Though a student may struggle trying to read the same level of books as peers, you can give him extra books to read during independent study or reading time. You can also use books at one and two levels below the targeted level as extra reading homework. You can use this method more stringently during long weekends, summer vacations and long breaks from school. Maintain a reading list for him including a mixture of reading levels.
Using age-appropriate books, have the entire class pair up in to teams of two. Make sure to put stronger readers with weaker ones. Have the kids work with one another by taking turns reading chapters or pages of a book. You should instruct the children to help one another and to not berate or badger each other. This exercise can also be useful in teaching children how to be helpful to one another.