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

read the last book of the series around 2005-2008. I think it is a trilogy.
I can't remember the name of the book, author or even the characters.
I just remember that the main character is a girl attending a boarding school, she is the only one with the power to enter the realm. The entrance of the realm is in the garden of her school. Everything inside the realm is very pretty, there is a big tree there. Ultimately she has to do a big sacrifice for the realm. Maybe she becomes the tree, but I am not sure.
Please, if you have any idea of what I am talking about, let me know. I would like to read the series.

Sounds like it could possibly be the Gemma Doyle trilogy by Libby Bray – the third is The Sweet Far Thing, and the first is A Great and Terrible Beauty. (I’m on mobile so can’t link, sorry!)

A Great and Terrible Beauty for the link to Tara's suggestion.

I think that's the one. Thanks!!!