I'm three grade levels behind in my reading than other students in my grade. Why is this? I don't speak English at home, but I've been in English-speaking classes my whole life. Can I catch up with them?
Even though you have spoken English your whole school career, it is not your first language. Your brain is a really interesting thing. Because you are bilingual, you don't realize it's happening, but your brain is constantly trying to reconcile what you learn in English with something you already know in your first language. This means while you are reading, your brain is associating the words and content with words and content you have previously learned in the language you speak at home. It is possible to catch up with your classmates as long as you're willing to put in the time to do it. Many times, when people start to fall behind in certain subjects, their willingness to keep at it diminishes. That ends up being the real reason that students don't always "catch up" with their peers.
I'm supposed to be writing an essay on student dress code. I have an opinion on it but not much information to back it up. How do I write an essay arguing for something that I can't defend well?
You don't. Unless you're running for political office, your actual opinion does not have to be what you write about. Set up a T-chart with Pro and Con columns. If you can come up with more of a defense for the Pro column, but your personal feelings lean toward the Con, write the essay from the Pro point of view. It is OK, and I would argue best practice, to write at least one essay from a perspective that is opposite yours. It teaches you to look at the other side of things, and even if it doesn't change your mind, it still allows you to see and understand the other side.
I have a hard time remembering concepts for tests or even future lessons. What's the best way to retain information I've been learning?
The key to remembering things is repetition. Find different ways to study the concepts you've been struggling with and set up a kind of rotation, whether it's in a single study session or over the course of days. If you know exactly what it is that catches you up, you can focus on those things more often than some of the things that come easier to you.