Presumably if SCC won't accept that she is year 5 then it won't just be Colchester County High school that she can't apply to in September but any state secondary school. Even if a school says they will take her early, if Suffolk LEA won't accept your CAF then you are stuck. The only options I can think of are:
1. Accept the situation and repeat year 5 or 6.
2. Appeal to Suffolk LEA
3. Go private, Woodbridge in particular are happy to take under or over age pupils
4. Ask Essex LEA if they would accept her as a year 5 pupil and if so quickly move over the border!
5. Complete year 6, as a year 5, sit the 11 plus in September 2016 and then home educate for the rest of the year.
I found this in the suffolk admissions policy. Has your case been to a fair access panel?
Applications for a place outside the normal age group
64.All schools are experienced in educating and caring for children with different abilities and social and emotional needs. It is expected, therefore, that children will normally be educated within their chronological year group.
65.Applications for children to be admitted to a school one year earlier or one year later than normal, to stay for an extra year with a younger year group or to be taught one year ahead will be considered carefully and the decision will be made on the basis of the circumstances of each case and evidence provided. Requests will be considered at the Fair Access Panel with the evidence provided.