I have just got a Statement for my ds1 (he's 10 next week and has AS). He in achieving well at school, in fact is well ahead of NC levels for his age in some areas, but "finds accessing the curriculum difficult due to the barriers his social and communication difficulties present".
That last quote is how you need to word your request for Statutory Assessment. I made it clear that, although I felt his grades could be higher and he is not performing to his potential in some areas, my reason for applying for a Statement was because ds needed ongoing support and additional training in social and communication skills and without this he would definitely fail to meet his potential in the longer term.
He also suffers from extremely high anxiety and support for this has been included within the provision on his Statement.
The sort of thing my ds is getting via his statement is regular contact from a sort of mentor figure (LSA) who will help him to organise his equipment, deal with homework (handing in and bringing home) help him complete his feelings diary and talk through any anxieties he may have, taking action where necessary to deal with any problems that may arise. He will see this person first thing in the morning, after each break and lunchtime and just before coming home. He will also have a weekly, hour long session with a teacher from ASD outreach, who will work with the mentor and ds to identify specific areas he needs to work on and help him develop strategies for dealing with them. So, social stories, action plans etc. She will also help him to continue working on his emotional literacy and regulation.
You will need to show that the school has tried various strategies to support him, but he needs more support than they can provide from their usual resources. It doesn't necessarily need to have been via IEPs, but they need to have 'evidence' of what they've tried.
I would put in the request for SA yourself as it really helps if you have the control and if its turned down, you can apply again straight away, whereas the school will have to wait another 6 months before they can reapply.
Start off by writing a list of all the things you feel your ds needs support with in one column and then what, in a perfect world, you would like the school/LEA to do to help. Write down every way you can think of that his issues present barriers to him meeting his potential and then how you feel these barriers could be removed or mitigated.
You will need a copy of the SENCOP (SEN Code of Practice), as its useful to use their language (hence the bit about 'barriers to learning', 'meeting his potential' etc). (Please excuse the website - its getting harder and harder to find a link to it online.) I also found a lot of useful stuff on SOSSEN, IPSEA and ACE-ED websites to help with what I should be putting in my request. Another one I've found more recently is Special Needs Jungle, who have actually published a book that will walk you through the process - too late for me, but it might help you.
Don't let anyone tell you what you can or can't do or what your ds will or won't get. They all have their own axe to grind. I was told repeatedly (and forceably) that my ds wouldn't get Statutory Assessment, let alone a Statement and we got both on the first try. The Statement we've got is badly written and needs a lot of amendments, but the content is worth its weight in gold to my ds and will be the difference between him sinking or swimming in secondary school.
Good luck. 