I can't talk about dyslexia, but for people who don't have that problem sight-reading is usually pretty hard too. How much time do you spend per day practising sight-reading? It's hard to improve at all unless you're doing consistent daily work on it. The rate of Improvement is usually closely related to the time spent, so fifteen minutes per day for a year will usually show less improvement than an hour per day for a year. It's also important to work on material that isn't too far beyond your current ability. There's little benefit in working on music that is written in two staves if you can't read one stave well.
Give us an idea of what you can do already and what you need to be able to do for your class.