What's the Name of That Book??? discussion

I've read this book more than 10 years ago so some things i remember about it might be inaccurate. I'll try to list every plot detail i can remember so there will be spoilers.

The book starts very simmilar to Harry Potter. There is a boy who is hated by his famili and is sent to a boarding school. He gets there by train, where he meets a chubby guy and a girl and they become friends. On the train they meet a priest. When he hears the name of the school, the kids are going to, he throws up his guts and dies. When they got to the school it turns out that it is not a normal boarding school. There is this cult that all the children are encouraged to join. All the children that are not in this cult ale not thought anything, they have no classes and and are just left alone. And all the children who are in this cult are taught magic. The protagonist and his two friends are trying to run away. The chubby guy eventually joins the cult and starts acting diffrently. The girl is abducted by the members of the cult and is forced to join them. The protagonist sees this but doesn't help her. He eventually becomes the last student who didn't join the cult and is forced to join them. After he comes back home he's changed and becomes cold and cruel. The book ends with him casting a spell that gives his dad a heart attack. As his dad dies, the boy stares at the black ring on his finger.

This is everything i can remember. I've beed looking for this books for months so i would appreciate any help.

Searching novel "boarding school" "black ring" "dark" this book came up: Groosham Grange by Anthony Horowitz (there's a second book too called Return to Groosham Grange)

One of the reader's reviews mentions the similarities with Harry Potter (even though s/he says that this book was written before the Harry Potter ones; it was published in 1988)

Hope this is it!

This is it! I can't believe you've found actually this.
May I ask where have you typed those phrases. I've been googling things like this for literally months.
Anyway, You're my hero. I can't thank you enough.

You're very welcome :-) I'm glad this is the book you were looking for!
I used Google too; did you use quotation marks ("") while searching? That's the key in many searches...