This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / Hogwarts Houses Divided
When Dewey speaks to his older brother's portrait
Dewey: Goodnight Cedric.
Another Cedric moment in chapter 3, when the Sorting Hat commented on Cedric's fate.
Sorting Hat: I remember your brother. I remember every student on whose head I've ever sat. Some of them have gone on to do great things. Others have gone on to do terrible things. And then there are those who never got the chance to do either.
Colleen and Connor.
Alduin. He ends up being shot when the Goblins take over Hogwarts. While he wasn't the nicest person at times, he still returns as a ghost to help protect Teddy and Deana from the Goblins.
Alduin: You'd better hurry. I'm sorry, I can't do anything more to help you.
It gets worse when Teddy and Dewey try to convince his ghost to return to the castle.
Alduin: I thought I'd see my parents. I wanted to see them so badly. Your (Teddy) parents are waiting for you. But they don't want to see you for a long time yet.
Chloe's eventual fate. She ends up being stabbed by the sword of Gryffindor while fighting the Goblins. While she lives, she loses her ability to do magic due to certain magic nulling impurities in the Goblin-forged metal. With that she has to leave Hogwarts and return to her normal 'muggle' life.
Violet's reaction to being invited to Teddy, Kai and Dewey's places for Christmas.
Cho breaking down in tears because she's so worried about something happening to Kai.
"I know it's not the same, but I figure he's as good as dead to us. Does that count as losing someone?"
"I never met my aunt, either. You-Know-Who killed her, before I was born. She and mother were really close, and she says I look a lot like her. Sometimes she cries just looking at me."