We had some similar problems with DD1. Things were difficult in reception and got worse in year 1, and peaked with a violent meltdown (chair throwing and kicking a TA on a school trip) in year 2. As a PP has said, although it's dreadful for you, it pushes the situation into a critical category that will force quicker intervention.
Taking good ABCs is essential, and was one of the things that helped with DD1. I'd recommend that you and the school both use them. There are various templates available that make it easier to set it out in a way that can be analysed later on. It's really difficult to keep them - both emotionally and practically - you find yourself thinking "hang on, did she hit her sister first or throw the ladder" 
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DD1 already had a diagnosed neurological problem which meant that when we sought help I think it was easier for us to get it quite quickly. We were referred to CAMHS and saw a neuropsychologist who assessed DD1 and then worked with the school and the LEAs ed. psych. They used a programme called Triple-P (positive parenting programme) which, when used consistently at home and at school, made DD1 much less anxious and aggressive. A bit later, they brought in the LEA Behaviour Support Team who set up a Positive Handling Plan which helped the teachers and TAs recognise triggers (which are identified by analysing the ABCs) and deal with them before the situation becomes critical. They also recommended setting up a social group which has been brilliant for her.
The other strand of the neuropsychologist's help was the cognitive assessment, in which she identified a very uneven profile and areas of thinking where DD1 really struggled and was getting very frustrated and confused. Once these were identified, the school was able to set up intervention programmes that helped, and indeed when DD was assessed a year later her profile was much more even.
We looked into having DD seen privately by a neuropsychologist. We were advised that the LEA wasn't obliged to follow the recommendations of a privately-obtained report. That swung it for us in deciding to use the NHS route, but if we'd had a fight on our hands to get the initial referral I think we'd have gone privately.
The chair-throwing incident was the worst it got. DD1 is now in year 3. She's doing much better academically. We had a multi-agency meeting recently and everyone is delighted with her progress. Her behaviour is no longer a problem at school although she does still need a lot of support. From the first meeting, when everyone was very defensive and edgy, to this meeting, where the staff quite rightly feel really good about themselves, there's been huge progress. We've had a horrible couple of weeks with her at home - I was about to post about it actually, but reading your post and realising how far we've come has helped 
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