My schools were all fairly relaxed by UK standards.
All of them had school uniforms, which you were picked up on if you didn't have the correct clothes.
Dyed/streaked hair wasn't allowed, nor excessive makeup or piercings.
The biggest issue dress-code wide at both schools was the length of girls skirts, since a lot of girls really tried to push their luck and having it very short.
But there was nothing security-wise going on. No sniffer-dogs, no metal detectors, but no violent crime anyway. There was a very small group of people who smoked cannabis in a secluded part of the school, but that was about 5-10 people out of 1000-2000 students.
Edit: The only thing regarding halls being patrolled was in my last school, where one person would stand at the entrance into the building after break/lunch, and yell at people to take off their coats now that they were inside.
That was the most pathetic and useless things they ever really did.