Overall I have to agree with you. In regards to ANYTHING she posted on twitter, so long as she did not use school property to do so, she can not be touched (and that has been proven in court numerous times the last several years.
As far as the first amendment goes however, what a lot of people fail to remember or realize (mainly due to poor educational standards across the nation) is that until you are an adult (or emancipated by a court of law) you do not have the same rights as an adult citizen. You do NOT have the right to free speech for example. You DO have the legal right to be heard which means if you have a problem or if you need to address something you have to be listened to and considered. You do not however, have the full freedom to say what you want when you want however you want.
Now, with that said, it doesn't really mean much at this point anyway because nobody is going to refuse to listen to a teenager or a child just because they are not an adult as well all know. BUT it does cover the fact that she does NOT have the right, and more and more schools are enforcing this, to say what she wants in school without fear of reprimand.
In regards to the school taking her phone and searching it? That is a direct violation of the fourth amendment (which minors DO HAVE FULL RIGHTS TO); and again this has been upheld in court numerous times over the past several years. This does not mean the student was right in recording the conversation without stating she was going to first (she does not need permission to do so, she just needs to state she will be recording so the other person(s) can refuse to talk while on tape). The proper and legal path has to be followed in order to obtain it, and under most circumstances it will be disregarded unless the recording was used to bring harm to the other person or is used as evidence and it will then be tossed out.
What it boils down to is if it did not happen on school property or at a school function, and it did not involve school equipment, the school pretty much has no authority. IF she is bullying someone via the internet and not from a school resource, then the only thing that can be done is to involve the police and have the police investigate the matter.
No Conrad I will not. If you are interested in following up on it you are more than welcome to read the constitution yourself, and the many, many judicial rulings in place regarding what is meant by the constitution.
After taking a look at your profile, and realizing that you are not (again this is going by your profile) an American citizen, and thus did not have the education I had growing up (Civics specific to the USA), I have decided to go ahead and provide a single link for you in regards to a minors rights being limited... See below:
http://family.findlaw.com/emancipation-of-minors/what-are-the-legal-rights-of-children.html
Minors also have rights under the U.S. Constitution. Specifically, they have the right to equal protection, which means that every child is entitled to the same treatment at the hands of authority regardless of race, gender, disability, or religion. Children are also entitled to due process, which includes notice and a hearing, before any of their basic rights are taken away by the government. - See more at: http://family.findlaw.com/emancipation-of-minors/what-are-the-legal-rights-of-children.html#sthash.TZeNhmXx.dpuf