1. It is pretty good I find that it helps you consolidate what you've learnt, as well as getting a little ahead of school. For me I really need to be TAUGHT things at least twice (that's why simply a private tutor who goes over HW is really effective for me), and in that sense it is definitely worthwhile giving it a shot
2. The max in a class is 13-14, but depending one what day you go it can be considerably small
3. This depends, that 'entrance exam' is more of a grading test to see what level you are at. they will then put in a class that seems suitable to your pace, so there are Accelerated classes, A, B, C. So it caters to everybody's speed pretty much.