IANAL and all, but I don't think the constitution is where you're going to find protection. The legal protection you're looking for probably exists, just not in the constitution. The constitution is kind of meta- as far as laws go, and you're looking for more specific local laws.

Policing kids for their social media posts is something of a new ground, and a lot of jurisdictions are just now trying to figure out what is and is not appropriate. I mean, you've got the need to protect kids from harassment and bullying, but you also have things like personal freedom and privacy.

There are school districts who have gotten in trouble for paying too much attention to social media before, and accused of snooping and spying. However, there are also schools that got in trouble for doing nothing when a kid was bullied into depression or suicide, and the district was accused of negligence. It can be hard to find a reasonable middle ground.

If you think things are out of line, my best advice for escalation is to start with your parents and see what they think. If takes a surprisingly small number of parents yelling at an administrator to get a policy changed. The next step is probably local media - reporters like stories about schools doing wrong, and school districts hate them. Finally, if that gets nowhere, and things get sufficiently out of hand at your school, it might be possible to get someone like the ACLU involved.

Finally, remember that as a student, your rights and liberties are weirdly limited. Due to a lot of "think of the children!" laws and public outcry, the courts and legislatures have generally have generally taken the attitude that school safety trumps student freedom. The primary responsibility of the school isn't your happiness, it isn't even your education - it's your safety. So if a school policy has to choose between your liberty or your privacy and your safety, it will be safety every time, and it will often have the law on its side.