I agree with Al1son. It could be that the support teacher doesn't have much experience of girls who have autism. They present very differently to boys and are much better at learning to mask the signs in order to fit in with their peers. In particular, their social skills are much more difficult to assess, unless you are experienced in working with girls who have ASD.
Tony Attwood's book is a good place to start and covers girls who have ASD and there are a few books specifically about girls here - although I can't personally recommend any of them, as its my ds1 who has AS. this thread on an autism discussion forum is an interesting read though.
Tony Attwood has made a dvd that might be of interest, details here - I'm not sure how available it is in the UK though, you might have to send for it from Australia.
On its own the 1-1 teacher's observation doesn't really mean all that much. Its just a snapshot of how your dd was during a certain time slot on a certain day. Its not possible to diagnose ASD on the basis of a single observation. Diagnosis is usually made after a comprehensive developmental history is taken, along with information on communication skills and possible sensory issues. In some areas the paediatrician will be able to diagnose, in others, like ours, there will be a multidisciplinary team consisting of Ed Psychs, Occupational Therapists, SALTs, Clinical Psychologists and a Paed. They will gather all the necessary information and then carry out an assessment (in our case it was the ADOS) before making a final diagnosis. The process can often be drawn out and lengthy, although in our case, it was the waiting list that took the time and the assessment itself was fairly quick.