London school: mixed, academic, small class sizes?

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Thinking about secondary schools for my year 4 son.
Wishlist is a mixed school with smaller classes which can really stretch him academically. Ideally the school itself would be larger so he can find people who share his interests.
He's not particularly sporty but likes drama, maths, history.
Prefer state but would consider private.
Somewhere reachable from central/SE London.
Any ideas on schools to consider?
Thank you

If you’re looking at a good state school you’re likely going to be limited by catchment so it’s only worth looking at those closest to you (unless there are any outliers that have other admission criteria).

Not sure if it will be feasible to find a large state school in London with small classes. Even the smallest secondaries (around 800 kids) in my area of North London have class sizes of around 28-30.

As @Tiredalwaystired noted, you'll likely find you're limited by catchment for most good state schools, and these can be tiny. Some highly sought-after ones around me have distance catchments of less than half a mile.

Gently, I think you have been led to some unrealistic expectations of state provision. Small classes are generally unaffordable in state schools and are restricted to those classes for the neediest pupils, and later on perhaps for less popular option subjects. Private schools tend to have smaller classes but sizes are creeping up as schools need to find financial savings.

Yep I'm just starting research really but that's what I had assumed. Might need to be private for smaller classes, if we can afford it.

Look at what is your local school(s) you may be pleasantly surprised. No point in finding the perfect school of your aren't in catchment (which can be tiny in densely populated areas) or you can't afford. You may find a decent local school plus a bit of private tutoring is a good option

Kingsdale?