My kids secondary they could get out of four or five exits. Getting in is harder, and you need buzzing in, but there's a push button for exit at all the gates. Actually the juniors was the same, so a child could walk out of that if they wanted to.
And the site is huge. Even if you knew exactly where the child had gone, it could easily take 10 minutes to get there and check thoroughly-especially if it was at change of lesson time.
Unless she told a teacher she was walking out though, then I wouldn't expect the first thought of a teacher with 29 children in front of them to think "Emily's later than everyone else, let's put an alert that she's walked out of school".
They'll have asked the class and probably got "She's gone to the toilet, miss". "She said she had an appointment" "she was in the last lesson..." "She was still packing her bags when I left." "I saw her talking to Mr C." So it could easily take 15 minutes to establish that she was missing.
Students do protect each other to a certain extent, and make excuses, assuming that their friend will turn up. I remember at school a friend's sister ran away and not one of the bus load told anyone that she'd got off halfway home with older boyfriend at the bus stop by the train station until police were involved. All sort of explanations were given-and she hadn't even told people not to tell.
Locally I had a young lad turn up at work who'd walked out of school and just kept walking. Last time he'd apparently been eventually picked up by the police 6 or 7 miles away. He was totally calm, quite happy for me to phone the school (who were very relieved). But he told me plainly that he'd asked his 1-2-1 TA if he could go to the toilet, walked in one entrance, out the other and left school. He told me that he had plenty of ideas to get out when he wanted to.
When school came to fetch him, I suggested they relaxed their mobile phone rule for him and allow him to put a phone in his pocket. They told him then and there that he could carry a mobile on the basis that once he'd walked out he rang them or Mum to tell them where he was.