When a child doesn't understand or been taught explicitly that the squiggles on a page represent the speech sounds of words, and if they haven't been taught the way the alphabet code works, then this is how the material they have to read looks. Gobbledegook with a few bits they recognise interspersed along the way. This is how the written word looks to children with reading difficulties/dyslexia. To work with the written word without solid instruction is confusing and threatening.
Imagine then their experience when they have to write sentences or stories? In the past week I have seen three separate examples of comments from teachers to students with diagnosed dyslexia. In each case the request was for longer sentences, an increased use of words and for better spelling! This is unfair. I understand that teachers have put in considerable effort trying to bring the curriculum to all their students but for those students who are confused and/or haven't been taught to read, write and spell by a teacher with a solid understanding of how our writing system works, how a child's brain works and how to bring the two together, then children will not be able to do what the curriculum and their teachers request - unless they are given specialist remediation.
This is also true in High School. I hear High School teachers talk about the needs of their students being for greater vocabulary and language. But if one looks closely the real need is for understanding of the alphabetic code and for someone to make the confusion understandable - because once someone has done this, the brain, with its innate drive for patterns and coherence, will take off.
Related to this is the cognitive ability testing that has been administered to children who have been diagnosed with dyslexia. If a child has been assessed as having average or above cognitive ability then initially one should start with the assumption that the child is capable of achieving grades of 'C' or above. (Of course this requires more sophisticated interpretation but it is a useful starting point.) Assuming some key elements of your child's cognitive profile are not too aberrant from the average range, if your child is continually being assigned grades of 'D' or below, even after they have put in considerable effort, then a conversation with the teacher about how your child's disability can be accommodated in the classroom is required. Average and above cognitive ability suggests your child has thoughts, ideas and knowledge, that they can express and which could potentially earn them a 'C' grade. Their disability will get in the way of this - but remediation will help and processes can be put in place to separate out its impact. Assessing their understanding and knowledge can be separated out from assessing the ability to hand write words and string them into sentences. It can be done. Simple voice to text, and text to voice features on IPads make a great starting point.