That must be a new thing. All three of my kids have attended public schools, and I can categorically state without fear of contradiction that they NEVER, NOT ONE OF THEM, ever had to "dismiss catagorically any possibility of religion" or "acknowledge atheism to pass exams".
Trust me on this.
Kudos to your school, too bad it isn't universal.
Safest way to avoid it? Don't give your kids to the athiest nanny-state.