The First Amendment protects your speech if you are speaking as a private citizen on a matter of public concern. However, if you are speaking as part of the duties of your job, your speech will not necessarily have the same protection.
Certain types of speech outside the school might also not be protected if the school can show that your speech created a substantial adverse impact on school functioning or that your speech was made in accordance with your job duties.
You may attend rallies on the weekend. You cannot be disciplined for exercising your First Amendment right to free speech.
If you attend a rally or walkout during school hours, your superior has the right to implement disciplinary measures.