I've been writing fiction stories for years
so here is my suggestion:
The question is, 'where she should go?'
-Since she is going to her high school, then perhaps the school can be in an isolated place like out of town, in the middle of forest, etc. Where she cannot go out or escape easily. Writing a fantasy, you can always make your own place... make your own world... your own continents... your own countries... cities... towns... you can all make it imaginary... because if its real... then people might think its REAL... ^^"
And if she has a plan to go out, then she will have difficulties or she might think twice because her parents will think something like she is not doing well in school... and of course.. if she say that she is not like 'them', then her parents might think she's crazy...
Next is the adding of fantasy
-To make your story original, i suggest you erase the warlocks in the list... (that my opinion though... if you insist on putting them, then its fine with me). And it depends on the type of power the rest of the students have. Vampires are fine with me...
and I also have an idea about this... why not... 'ghosts or phantoms'?... since they are ghosts, AND students... the place they died is at the school (of course!) then all you have to think about is WHY they died and HOW...
Does what you have so far have enough body to start?
-For me... no
When writing ANY kind of story... the writer must always have the answers whenever the reader has one question like... 'what happened?'...'why did it happened'.... 'how did it happen'... and so on... and now, like a reader like me, just posting the small details... there are some questions that popped up in my mind.. here:
"How did they know about the school?"
"Why and who decided to let her to go in that school?"
"Was her eldest sibling went to the same school? Yes or no and why?"
and... "What inspired you to do the story?" just want to know... ^^"
You can think about my questions... (its alright if you don't post the replies... so its a surprise
) but if you have it... then ignore ^^
hope that helps