How did the principal "make" her walk home? Did security escort her off school property? When I was in high school (hell, even elementary school) I was allowed to walk home whenever I wanted -- it wasn't the school's responsibility to make sure a parent/guardian picked me up or I took the bus. If I wanted to walk home I just left. I regularly walked home from school, and my high school was 2-3 miles on roads that sometimes didn't have sidewalks.
So did he say, "Go now, you cannot stay here anymore." And she said, "But it's dangerous out there!" and he cackled and sent her packing? Or did he say, "you're free to go now" and she said, "but I don't have a ride!" and he said "Guess it's gonna be a long walk then." Because I see nothing wrong with that. If she didn't feel safe walking home couldn't she have sat around in the lobby and waited?
Personally I don't see the whole walking home thing as an issue. I'm not sure I see the "ignored threats" as an issue either -- how many hours did she give the school to address it before she went and got in a fight over it? If she really felt threatened, I feel that she should have either called the cops, or sat around in the administration offices demanding to be protected. The way it played out it sounds like she was mad and went off looking for a fight. Which is understandable, but I'm not gonna fault the school for failing to protect her if she didn't do everything she could to protect herself.
The length of the punishment does seem egregious, so I'll give you points there, and I'm glad that you're not playing her out to be the victim. If the school hasn't seen the text messages yet then they need to, and hopefully they will result in more punishments being doled out in the other direction.
I think you're right not to go to the school board right away. See what the guy has to say and let everything sink in and see if you're still upset after the weekend.