My DD is current year 8 and about to sit common entrance for entry to senior school into year 9.
From our experience so far I can tell you the following. Its not gospel across the board but its what we have found in MOST senior schools we have considered:
Year 9 is a fairly natural intake time. Alot of senior schools either start at year 9 and those that start at year 7 also have an intake into year 9. Several schools we have looked at with regards to Yr 9 entry (that start yr7) have special integration programmes/trips/schedules arranged to ensure all new yr9s settle in ok with the existing peers in their year group.
Most children enter into Yr9 through the Common Entrance Examinations sat in early June in year 8. Most if not all children sitting CE have been prepped for it through their prep schools. Usually, the examination week takes place at the current prep school BUT most senior schools can and will make arrangements for the exams to be sat at thier school if neccessary.
Most schools we have looked at also say that they also have their own entrance examination papers for entry into year 9 by those pupils not currently in a preparatory school/being prepared for CE or from overseas. These examinations are usually taken earlier in the school year - typically in the Spring term but varies from school to school.
Most senior school require candidates to attend an interview with key members of staff Head of school, head of year etc etc.
If your children are gifted in any area academic, sports, arts, dance, Music, sport etc etc. Then they may wish to sit scholarship papers. Again these are usually sat in the spring term. With regards to scholarships in specific areas such as Art for example: the candidate will be expected to bring a portfolio of thier art work and talk about it at length and then usually spend a day or an afternoon taking part in an art excercise/assessment at the school.
You will find by looking at most schools websites under Admissions alot of useful info about their admissions procedure. The schools have registrars or admissions staff who are there to answer all the questions you have so ask away and explain your circumstances, they will advise you accordingly.
I would suggest you get intouch with schools and start to look now really as some schools have lists that close once full. Also, if you are avroad, you are going to need time to come to the UK and have a look around and sometimes one visit is not enough. We sometimes found we liked the school side lots but were not so keen on the boarding house etc or vice versa. So please do start looking now.
You may find these websites helpful in your search for schools
www.isc.co.uk/
www.isbi.com/isbi-searchform.asp
www.gabbitas.co.uk/
www.sbsa.org.uk/
Good luck and let us know how you get on.