Once you have signed a permission slip you have given permission. I can't comment too much more without seeing the way it was worded though. Read before you sign and if a choice has been provided make sure that you have checked what you want.
If the parent refuses to grant permission then permission is not granted and the student can't do whatever it was. It is common for a few parents not to grant permission so I don't know why they would go ahead anyway. It is not 'illegal' but the school can get in trouble.
Putting on my ex-teacher hat for a moment I would suggest that you talk to the person who collected the permission slips and ask to see your son's. Children have been known to 'loose' the original permission slip and to substitute their own. With something other than what the parent wanted indicated.
If the correct permission slip is on file and it was filled out how you wanted, then the school is in trouble. I smell a rat because schools/teachers are, and have been for very long time, respectful of what the parent wants.
