I'm not sure exactly when CSSE informs the primary school, but if you checked the 'results not to be divulged to school' box, then the answer is 'never'. If you did this, you will have had to set a password to be given should you need to contact CSSE about your application. When the email results system went wrong last year, I think quite a lot of people regretted taking the line that it was none of the school's business, as those who had opted not to deny the school the information were eventually able to find out their score from the school.
Yes, this happened to us last year. We got the result from DS' primary school a full 24 hours before CSSE managed to get the results email to us.