Are you angry and hostile? Or are you maybe just a little frustrated with life?
Tell mom you are not a phychopath, you are not angry or hostile. Sometimes you have a hard time expressing your feelings appropreately, but you are not delusional, vindictive or "crazy". Tell her that while you know she worries about you, it hurts your feelings that she does not understand that about you.
Being a loner does not make you a killer. it just means you are "content" to be along. Point out to her how you want to "help" people, and go out of your way, in your own way to do so. You don't want to blow up schools, shoot peoples heads off, or harm anyone. Again, tell her that is hurtful she would think such things.
As for school, just be careful that you don't say "scary things" to people. I have heard of men who talk to women, and in trying to seek attention, will say things like "ever thought how you would feel being tyed up against your will" or "I have a gun". And walk off, enjoying the chaos they know they cause.
Be pleasent, quiet, aloof, yourself. I would not go out of your way to behave "differently". If people talk to you, do what you can to "at least particicpate a little". Just exchange a few pleasentries, and that is all. "hows the weather eh?" "not bad, sun is really shining!" "yup". I don't see the need to go a lot further then that...
As for the school, I don't think it is a worry. Are you writting manifestos or saying scary stuff to people? Or are you just by yourself most of the time? Do you have a track record of violence? Strange encounters documented by the administration? If anyone asks you from the school side if you are "ok" tell them yes, and ask why they ask. If they say that it is because you seem so quiet, then perhaps, with the staff, tell them about why you are quiet (neurological) and that you have a hard time fitting in, don't really want to, it is not that you are angry or hostile, or don't like other people, you are just an introvert. They should accept that.