I agree with Saintly.
DD1 can be an aytypical communicator, and tbh, I'm not even sure her SS realise it. That is because at home, she'll be very adventurous with her use of language, and we take the 'long way' to understanding her. She'll say 'long yoghurt' for Frube, or 'it pink and go weee' for slide, for example.
At school, I suspect that she sticks to safer topics where she can express herself. I know they find it hard to understand her at times, but then lots of adults in general who don't understand something simply fade it out until they reach a bit they do understand, don't they? That's why we can decipher some foreign conversations, because we can latch on to the central words, use the context we are in, and ignore the detail.
For example, I am in an on-line group with a multi-lingual make up. All members use English, some better than others, but there are around 8 or 9 different languages of origin. There was an argument in group, and one person was sticking up for a largely marginalised group within the group. It wasn't going down well, because the person in charge didn't want to hear it. One person got badly attacked and another then resorted to defend them, by using Swedish. Now, I've never heard Swedish, never read Swedish. But by the context, and the fact that I could decipher that 'unika' was 'unique' and some other words, I could tell that the person was saying 'you're attacking someone who is important to this group. I don't know what your agenda is, but I've got your card marked'.
It sounds like your DS's school haven't tuned in to your DS. In fact, it sounds like they haven't even reached the stage I illustrate above - they're not able or willing to pick through the 'white noise' of stuff they don't understand to find the gems of stuff they can understand.
So, you can either focus on the school, or your DS. In the long run, your best approach would be to focus on your DS.