I like the theme and that the boy has troubled background and that the story includes
a witch.
For a story you will need a problem / the characters need a goal.
Example:
The boy´s problem is that he is not popular in school and the prom is coming
up, he is the only one who doesn´t have a date.
The witch girl´s problem/goal is this: she is sent to the human world to put
a curse on a human to fulfill her final witch exam and be able to be a real witch.
The conflict of the story is that the witch girl starts to like the boy but witches
can´t be friends with humans and being friends with a human with exclude her
permanently from the witch world.
You can use the witch magic for all kind of situations in the story (for the bullies,
the teacher, the abusive father etc.), I would not write a witch girl who can´t do magic
because readers who are reading a story about a witch want to see some witchcraft 