A is far better than B, then students at B have no chance at all;
Most important is the admission history: If A is supper good and is the feeding school then students have more chance. If B is reasonably good and have good admission record, your kid can top in the high school then he/she has better chance. If B is not very good and no good record then the students have almost zero chance.
So, firstly, good high school admission record; secondly, the rank of your kid.