Let's look at what happens if kids are given full, unrestricted access to the basic human rights, either in the US Consitution or the international treaties like the ICCPR. In particular, let's see what happens if those rights are abused by school pupils:

Freedom of association: Kids are allowed to bring their KKK or neo-nazi skinhead uncle to school for company.

Freedom of expression: Kids get to write essays and projects that are full of profane and pornographic content, and the school can do nothing to stop them. Similarly, at any time during class, a kid can stand up and start yelling verses from the Satantic Bible.

Freedom of movement: They can just stroll away from school whenever they want, thereby getting no education and no discipline.

Freedom of thought and opinion: Kids already have this. Schools can't censor what you think, only what you express. It's the same as the adult world.

Right to bear arms: More innocent kids get slaughtered. This happens too often already, with 10 American children getting shot dead every day.

Freedom from arbitrary arrest or detention: Schools need a specific law entitling them to impose detention on troublemakers, and kids have the right to appeal their detention through the courts.

Right of peaceful assembly: Kids can get together for a meeting with each other in the middle of class.

To put it simply... if you give children the same rights and freedoms, to the same extent that adults have them, then you end up with an entire generation that is both uneducated and undisciplined. In order to teach, schools need the power to force certain behaviour from their pupils.