Autism presents very differently in girls as compared to boys and many of the 'signs' that people mostly wrongly attribute to autism centre on the male presentation. e.g. lack of eye contact, little empathy, lack of friendships, unusual obsessions.
Generally girls mask so much better than boys when they are at school or socially, which is why teachers and sports coaches don't notice them presenting much differently to their peers.
why it's taken so long for professionals to diagnose
When did you first suspect she was autistic and ask for investigations or interventions?
The NHS diagnostic process can take a very long time from raising your initial concerns. Again, depending on who was doing the assessments and if they were aware of ' female' autism. There's an awful lot of ignorance about autism in general and particularly about its presentation in women and girls. Smallblanket is not alone in being wrongly fobbed off.
Some women aren't diagnosed until Uni, others later on when for example they find the inferences and unseen communication aspects of work difficult to cope with, or even into middle age and beyond when they have known for decades that they are very different to their peers and would like to know why.
The upside is now that your DD has her diagnosis, you can request interventions are put into place at school to help her cope. Do see the school SENCO before schools return and see what school can offer her. Some schools are great, others are rubbish, if they aren't helpful, do consider finding a school that is.
Not sure if you're aware, but if she appears to be immature for her age, many kids with AN have an emotional age about two-thirds of their chronological age, which can explain quite a lot of their social difficulties when compared to their age-peers.