SE London Secondary options overwhelming - US expats need guidance :)

OP have you been prepping for the 11+ exams? The likes of Alleyns are competitive. You can’t just turn up and expect to get selected.

Deadlines for registering DC to sit entrance tests for various South London state grammar schools, from the relevant websites:

London Borough of Bexley: 31st May
St Olaves: 1st July
London Borough of Sutton: 2nd August
Tiffin School: 2nd September @12 noon
(so that one is still a possibility)
https://www.tiffinschool.co.uk/admissions/year-7-admissions/

Rather a traipse from most places with a SE postcode, though.

Unlike with indies, entering the DC to sit an entrance test for a state school doesn't constitute an apication for a place - you still have to name the school on your Common Application Form to do that.

Co-ed indies easily accessible from West Dulwich:
Emanuel
Westminster
Thames Christian

Grammar schools:
St. Olaves

State schools:
Kingsdale (no catchment, lottery system, you can try for a scholarship as a PP suggested)
Norwood School (catchment, unless applying for a scholarship place)
Charter School (tiny catchment)

This should give you an idea of the options:

List of school open days

St Dunstan’s and Alleyn’s seem to me to be rather more glossy than down to earth schools. I know they have recently installed gender neutral toilets at Alleyn’s and a few years’ ago the head of St Dunstan’s was forced to backtrack when he invited a paedophile apologist to come and speak to the children during Pride. These things may not bother you. They would me.

State options depend on where you live, unless you’re applying to Kingsdale which is a lottery system (although you can game it somewhat if you get a maths scholarship).

Coed options roughly near Oakfield I can think of are:

Charter East/Charter North
Norwood School
Graveney
Dunraven
Elmgreen
City Heights E-ACT academy
Trinity Academy (not the boys school in Croydon)
Bishop Thomas Grant school.

I don’t have personal experience of any of these schools.

South London grammars are in Kingston, Sutton and Bexleyheath but are single sex.

Unfortunately there is no shortcut to doing your own research as you know you what’s doable for your family, your child, and the kind of education you want and can afford.

11+ would be grammar school options so do look into those state school options as well.
London independent schools especially coed are competitive to super competitive. The main point to check is travel time, not distance. We found some schools that looked further away but has train, tube, bus options or car share that was better than those that looks nearby but due to traffic and building or road works would take longer and only have bus options. Cycling depends on what your DC is carrying or musical instruments and school music commitments.

If you haven't done so already, go to the secondary school admissions section on the website of the local authority you live in and read through all the information relating to the process.

The national deadline for making an on time application for a secondary school place is October 31st.

You submit your application through your home LA, but can include state schools located in other LAs. But be reasonable; look at the published admissions policy for 2025 / 2026 for each school, where you will see how applicants are ranked if the school is named as a preference by more people than there are places available for entry in year 7. From this, you can see under which criterion your DC would be ranked, and either the LA where the school is situated, or the school itself, should provide information as to where the last offer fell in recent years (but obviously, this can change from year to year, depending on the characteristics of the cohort applying).

If you are applying to state schools you do so through the council. Be aware of the deadline to get application form in, and find out when the open days are for the local state schools. You make one application and put the schools you are applying to in order of preference.

Where are you in SE London? is your child going to take the 11+ exam?

When it comes to state secondary school you are limited by your house location. Look at the catchment for schools around and you will see if you have any good state schools around. You will find it in the admission section of each school.
No, you don't have a wide choice of SE state school. Only the ones that you live by