I know Zakryn may have overreacted Chad, but we all have our own opinions about what is and isn't fair punishment.
When Tobias vanished, the school will have called his Uncle, who told them he'd moved. Nowadays the school probably would've followed up on that (or at least got angry at the Uncle for not telling them in advance that Tobias was leaving) but maybe not at the time the booms were written. Or maybe they did and that's how they found out he wasn't staying with either of them.
As for Marco and Erek, they were never friends as far as I recall, just guys who knew each other. Marco said a few people from school turned up at his mum's funeral and Erek was just one of them.
Oh, and Melissa may not have been an Animorph, but her dad certainly deserves considering for worst parent. He bloody told the Yeerks about Earth in the first place! He sold out humanity to the Yeerks and got a slug in his head in return. The fact that he doesn't remember any of it doesn't excuse him. The fact that he seemingly grew up to be a nice does redeem him slightly though (he became a Controller to save his daughter).