Oh Stary - I had exactly the same experience. DS2 got a place at the special school before we'd really got into our ABA programme (though like Cyber, it was 'ABA-Lite') and although I thought it was a great school (and still do) I had more and more doubts as time went on about how well he would learn in that setting, as the ABA programme had shown that he only really learnt through intensive 1:1, prompting and reinforcement, rather than TEACCH type methods, which was the approach used mostly by the school. Also, the class he was in was, at the time, composed of children who were(like DS2 at the time) not at all interactive with their peers and minimally verbal. He spent two terms there part-time (with a part-time home programme) and we then transferred him to part-time m/s (with the full support and encouragement of the special school)on the understanding that he would respond better to a full-time 1:1 and to children who would model language and social behaviour to him.
The original idea was that I would teach ABA methods to the 1:1, who would then use them in school - they seemed open to this at first but it hasn't really worked out in practice, as they refused to use edible reinforcers, which were the only things he would work for at the time! I have managed to sneak quite a lot of ideas in through talking with the 1:1 and the home-school book, but I would say that, on balance, he probably hasn't progressed as much academically as we'd hoped.
His social skills have come on hugely though - from taking no notice of other children at all, he now has a small group of real friends who he recognises, responds to and initiates contact with. He also interacts with a wider range of adults and his sensory tolerance has improved enormously.
Have you thought about a part-time programme at all? You are allowed to 'flexi-school' if the head agrees and you can prove you are engaging the child in a valid education off-site (though they never inspected me!)
Or, if you feel after a while that the special school isn't right for DS, what are the alternatives? Nothing is set in stone, and if there is a viable choice, you can change things if they're not OK.
I found I had to make a huge leap of faith to send DS2 off to school and it took me a long time to relinquish control (I still haven't really, am in there all the time and am sure they've got a picture of me on the staff room wall they use as a dartboard ) But atm, though it's not perfect by any means, we've found the 'best fit' for DS2, even if it's taken a long time ( and MUCH soul searching) to get here.