The girl who rescued you from the toilet is a hero!
If your ex is narcissistic, nothing anyone says, not even a judge or jury, will make a blind bit of difference. Even if he seems to come around in some tiny degree on one action, by the time you draw a breath, he will already have rewritten it in his head to make it even less his failt or problem than it was. He will get worse and worse. You must continue to block him from everything you can, and hope he finds another target.
It is possible to have your screen name be different than your 'real' name on facebook. On Facebook, do a search for different arrangement of your name and initials, and use whichever one has multiples as your official name. List the absolute minimum of other information, or even put nonsense like you live in the Night Garden and you attended school at Hogwarts or some other fictional school. You can build your persona with whatever you like, really. If anyone ever questions it in future, say it is done for security and privacy. All of this can be changed later, if you feel safer.
Then create a nickname. It can sound something like your name or nothing like any name at all. It can be related to the nonsense above, or not. Make it short & easy to spell though, so classmates can recall it. This is the name you share with trusted people. It will be searchable to the degree your settings allow.
Don't put your picture for the avatar, put a picture of some art creation of your ds or yourself. Or your ds favorite animal. Or yours.