I read a book about all of Loach's films.
Originally the kids were told they would not be hit and it would be shot from behind to make it look like they were hit.
2 mins before shooting the scene, the head is told to hit them......Loach said it was the only way to get the authentic response from the kids from being hit......
Once the original shoot was over Loach wanted to do a few more takes and the kids refused - but as you correctly said, they came to an agreement for 50p for each further whack (the kids were on £11 a day) - all apart from the little kid who had only gone there to deliver a note. He was exempt from further whacks.
Fantastic film that I never fail to watch once a year. Remember showing it to my kids when they were at North Manchester High School for Girls and they didn't believe that us kids going to school in the 60's and 70's were treated to such brutal regimes