I think I posted my thoughts on Emanuel last year when we went to Open Days and it was one of the schools I registered DD for (in the end decided not to sit the exam) and my impression was that they didn't pull their socks up academically.
In order to do that - they need to do one of two things 1) increase selectivity of intake; 2) teach the relatively mixed ability intake better and get the most out of them. My impression was that they do neither very well.
My recollections have now faded, but one example of 2) not being done very well is that they don't set in Maths for TWO years, i.e. Yr7 and Yr8. This is ridiculous for a school that is not very selective. I can understand why JAGS, Alleyn's, SPGS etc don't set until year 8 or at all but even KGS (which is definitely more selective than Emanuel) sets after Easter - into 6 sets.
The best state comps set maths, often from the very start.
Then they actually acknowledged that they kind of stream into to streams on entry - A and B stream based on entry exam results. So if you are great in maths and a brilliant scientist but bad at English you may end up in Stream B! That smells of sheer laziness to me. But wait... It gets even better - the streams are kind of correlated to the language you choose - so if it's French you decide to study, it's top stream and Spanish/German (I forget which it is in Yr 7) you are in the bottom. May be somebody sees the logic in all of this - I don't.
I didn't like the headmaster - he was complacent, defensive and generally not particularly available for chats before or after the talk (unlike in all the other schools I visited, including JAGS, Alleyn's and KGS).
We looked at Woldingham as back up - it's only mildly selective (probably less so than Emanuel even) but they set in Maths straight away, and band in humanities and sciences. Their results are way better than Emanuel.
Also, Emanuel is very much viewed as plan B by many (still I believe) and really academic kids often choose to go to a more selective school even turning down 30/50% scholarships.