Reading is the gateway to all learning. If you can't read you can't learn. Reading occurs in every subject, so if you are having trouble reading, then you might struggle with every class.
When reading it is important to have the right book, meaning it needs to age appropriate. If it is not appropriate, it will not challenge you, it will not help you grow as a reader. If you don't grow as a reader, you won't grow as a learner.
The following list are links to books that are on the 8th grade reading level: