I can't judge this school too quickly. All three of my kids' schools have a policy of calling the parents when a child isn't there and all three have had an incident where I wasn't called.
1) high school - my son had been very sick for a few days, I had called the first 3 days and on the third mentioned that we were going to the doctor. On the 4th day we slept and I forgot, but no one called me b/c I had been speaking with the attendance secretary every day and she just figured he was still sick.
2) middle school - similar to this situation. End of school, crazy day with parents in and out, exams over and it was just the fun stuff like the talent show, etc. LOTS of middle schoolers not attending these days. He wasn't there and they didn't call b/c it was about half the middle school they would have had to call.
3) elementary - they normally call but my DD was sick and I forgot to call - same day they took a child by ambulance from the school. The nurse was dealing with that and never got around to her calls to the parents.
I can see how at the end of the year the routines are not as, well, routine. Yes, someone could take advantage of that, but it would have to be someone who knew the situation. In that case it is probably an "inside job" and a person who would also have many other opportunities just b/c of their knowledge of the school.
Let's find this baby and figure out what happened before we put the step-mom or the school nurse in jail!