Agreed with Rohit. Schools with weak cohorts tend to give prompts/topics out prior to the SAC itself. Everyone will will/should perform better than they would if they went into the SAC without knowing the prompt/topic.
However, when faced with a situation (like the exam) in which they need to write without knowing the prompts/topic, most people will be completely stumped.
If you want to avoid falling into this trap, I'm under the impression that you need to be Rank 1 in SACs and perform just as well on the exam as you did in your SACs.
For example, I go to a private school with a relatively strong cohort. Nevertheless, our second SAC was a Context piece that we were given the prompt two weeks beforehand and allowed to write at home and hand in. Many people got very high marks (even though half of them probably had it written by their tutors!). However, they will fall away after our third SAC (which is this Wednesday) as we haven't been given the prompts/topics for that.
Hope that clears things up further.