Any student in a public school is guaranteed by federal law the right to attend school without being harassed. Hitting her (which can be viewed legally as assault if we want to get technical) and verbally abusing her would constitute harassment in anyone's mind. Notify the school principal and superintendent in a face to face meeting that you will not tolerate any further persecution of your daughter. Give them a fair amount of time to show their concerned response. I would suggest they bring in this boy and tell him to repair, as much as can be done given the damage he's caused, the harm he has brought about.
Meanwhile, it certainly would be encouraging to your daughter if any friends she has would show their courage and compassion for her, yes even in the face of being unpopular with the cool kids, and stick up for her in school, walk the halls with her while this is cooling down, and so on. Kids probably do know he's lying and some kids obviously know they can destroy your daughter's emotional well being.
She's going to need your support, understanding, and perspective during the coming weeks and months because her school and what she has been turned into there are both horribly embarrassing to her right now. Don't just tell her to shake it off and ignore it all. Show her through your empathetic language that you know how much pain she must be in and don't spare those hugs and private talks alone with her. She will recover but she doesn't believe that now. If she keeps getting worse emotionally (over a period of several weeks) please consider getting her a therapist with whom she could talk. Don't assume that just because she isn't talking with you about her pain, that she is not in a great deal of it.