My DD is currently at MS in Y7, but tbh, hopefully not for much longer. I've thought she should be in SN school since she started YR, LEA have taken a lot more time to start to agree....Thing is, if your dc needs A LOT of help with the work in MS (as mine does), there just isn't the ability in the timetable at most MS secondary's to give as much help as is needed. My DD has dropped French already in order to be able to fit in the extra maths and english help she needs, but the school has everyone, even the lowest set, take French/German for GCSE. Which DD will be guaranteed to fail if she only starts Y7 work on it in Y10...if she can manage without the extra maths/english help by then. Which she wont. I'd have rather had my DD go to SN school from Y7 as there will always be more SN dc's moving to SN at the start of Y7, so even if not with friends, there is a chance to make new ones.
Maybe the SN school you are thinking of is not right for your DD, is it a school aimed mostly at physical difficulties, SLD or MLD? Finding the right SN school can be just as difficult as finding the 'right' MS school. DD's MS school was chosen purely on the basis that they offered the most SN help of all the Secondaries in my town. For the SN schools she will (probably, by all accounts) be moving to for Y8 I have a choice of 2, one of which is much more suitable for my DD than the other.
I think it would be much better for your DD to start SN school in Y7 with other 'new' children, as she'd have to get to know new dc's even at MS, my DD ended up in a form class with ONE other girl from her year from primary, who actually disliked my DD. . At least your DD would not have to sit through the first term of Secondary not understanding the work until the CATS test results come back and not being given enough help until October/Christmas.
Maybe I'm biased towards SN schools as I know that would be the best environment for my DD?? I would say though, when looking at MS secondaries, do bear in mind that the provision for SN can vary immensley(sp?) from one school to the next, and it is worth speaking to the SenCo at the open evenings and pinning down what the current level of help available for a dc at the school in your dd's situation would be/ what help they would offer your DD. My DD in Y6 was still 'only' on SA+ (now getting statement hopefully), and the difference between the local schools varied from 1.5 hrs a week at one school, to 8 hrs at her current school. Where she is actually now getting 11+ hrs a week at!
ASK LOTS OF QUESTIONS!!!!