iLoveChubby:Shan:Yes, I agreed that kids of similar abilities should be put together.
It does not work to have weak & high ability students together.
But I dun agreed....that centres/tutors are good cos they have high percentage of A & A*, especially when they select/accept mostly high ability students.
Peace 
I am with you on this.
Probably tracking the improvement of grades will be a more realistic measure on the effectiveness of the centre/tutor.... although there are many out there who are in enrichment classes not to improve but to maintain their A*s.
Yup, so true..I know of friends who used to go to centres who only take good students, after a while they realise while their kids still do well, there is no value add..good centres are those who bring an improvement, not just a maintenance.
Granted, for those getting A*s, it might be hard to see the value add but one of my friend started the migration from these centres which allow only the better students in and joined a lesser known one for Maths and English, for Science, she got a personal tutor. And her daughter not only improved in her A* grade but for Science, started to really love the subject, for English, she begin to really enjoy the process of writing and write in a less manufactured way. Then, the rest of my friends followed.
So to me, I am not so taken in by a centre's advertising of its good grades, I look more at their programmes, and the way they teach and the extra value add.
But to be fair, I do have a friend who stayed at one of the top centres which takes in the best and she finds it beneficial. So who knows..But, to me, it is better to thoroughly look through what you are signing up for before just focusing on the results advertised.