See, I'm not sure Andrew would've been aware of this. He reportedly had a 100% perfect attendance so he had no experience being absent. I imagine the procedure for reporting an absence was discussed with the parents, since they'd be the ones making the call. But would the school have discussed this with Andrew too? I honestly don't remember if it was discussed with me when I was a high school student.
I think it varies by district and even the kind of school too. I went to private school in the '90s, and in my case, if a student hadn't shown up by the end of the first hour of school, and no one had notified the absence, the school nurse would call the parents to inquire. I think I was aware of this, but I never thought about it since whenever I needed to be absent, my mom would call.
I don't know what the procedure was like in public school, and I certainly don't know what it was in the UK in 2007.
My hunch, if I put myself into Andrew's shoes, is that he was unaware that the school would try to call his parents. If he had known, then his actions might've been different. For example, he himself could've called the school when he returned home, and there was nothing the school could do since his parents had already left for work.
I'm sure he knew he'd have to explain his absence at some point, but he just wasn't thinking that far ahead. After all, it was a Friday, so he wouldn't have to deal with this till Monday. He figured he could just lie and say he was sick, and the school would believe him since he was never absent.