I was you once, complete with speech therapy and I remember the sick frightened feeling. My son was slightly older as we don't start school till five and every time his teacher saw me she told me new things about his shortcomings - often while crossing the playground. I still remember her chirping about his inability to catch balls having something to do with his bad spelling. I looked askance at this point as my spelling was fine as a child and I couldn't catch balls very well either. I changed his school. The new school was much better and he ended up with a teacher's aid and in reading recovery.
My son is now twenty and at university. To be fair, he never turned into a great student and he has chosen a degree that does not involve as much academic rigour as some others. It will not be an entirely brilliant degree. On the other hand, he has very good high school exam marks - our equivalent of your A and O levels. His speech is clear and he recently got his driver's licence.
My son had ADHD and a hearing problem. He was very hearing impaired - not totally deaf. He fooled a whole lot of professionals - creche staff, family doctor, speech therapist - about his ability to hear. We think he taught himself to lipread. He got grommets and medication to help him concentrate.
I do urge people to have a hearing check if there is a speech problem. My son co-operated beautifully during a hearing test and failed completely. Luckily there was a cancellation and he had the grommets in within a week of his appointment with the ENT specialist.
I found a book dealing with ADHD. It was called "Right-Brained Children in a Left-Brained World : Unlocking the Potential of Your Add Child" by Jeffrey Freed. Thanks to the techniques in that book my son's spelling list was no longer the seven day marathon it had once been. The book explained about other things such as the need to give short instructions rather than a long string of instructions where they forgot the beginning bit before they'd gotten to the end. I had read many, many books on the subject but this was the one that actually helped. (I promise I have no relationship with the author at all and this is not a paid commercial.)
My son had a lot of personalised tuition which was effective. We chose not to send him to a private school but had a mixture of paid tutoring and us doing it over the years. Basically we covered the whole school curriculum with him after school and at weekends. His teachers were very helpful with providing an outline of material that was going to be covered in class and we bought a whole lot of "cram" books. (His teachers were co-operative because they realised they then had a child who was keeping up and they didn't have to spend the extra time or answer awkward questions about why a child was failing. ) You have to make the lessons interesting and relevant. For example, you might talk about reactive gases and explain that hydrogen has only one electron and is unstable because it's always trying to find an extra electron to hang out with to fill its shell while helium has two electrons and has a full shell and doesn't need anybody else. You might share then that unreactive gases these used to be called the noble gases because they were too proud to hang out with other elements. I then showed him footage of the Hindenberg disaster - a hydrogen airship (there are no gruesome close ups and actually the majority of the passengers somewhat surprisingly survived). He never forgot the idea that helium was non-reactive compared to hydrogen. You might explain that Germany used hydrogen because in the pre-WWII years, America didn't trust Germany and wouldn't make helium available etc so you include a bit of a history lesson. ADHD children need constant revision as well. Asking the child to teach you the material is also very effective as revision.
Obviously this advice is tailored to an older child but the principles are the same. I turned up for every parent teacher meeting and I quizzed them. Teachers often won't be direct and you don't want to find out half way through the year that your child is completely lost - but in your case your teacher seems to have made up her mind on day one. Interestingly, and maybe not unrelated to my son's issues, I ended up in the slow learners' class at school when I was about 7 or 8. They were the best school days - I could do the work much quicker than my classmates, I'd secured a warm seat next to the woodburner and I spent my days reading books. It was idyllic till they must have realised I didn't belong there and biffed me out of the warm pre-fab in the gravel carpark out the back and back into the regular stream. I have two university degrees in hard technical subjects so I probably didn't really belong in the slow learners.
I don't mean to suggest my son's high school days were one giant swot session. He did have parties and outings and so on. I am not saying that your son has ADHD at all but I think some of the techniques in the book might be helpful regardless.