What to do if you know you're in trouble:

Just Google “Teacher Falsely Accused” and you’ll find evidence that even the best of us can find ourselves on the wrong end of an administrator’s, parent’s, student’s, or even another teacher’s unfair, inaccurate, or patently false accusations. If this happens, you have protection and a union to fight for your rights.

Even if you did something worthy of discipline, you still have rights, and it’s difficult to fire a teacher without just cause. Furthermore, thanks to unions and union advocacy, most issues are required by the contract and state law to be formally documented and remediated before a school board dismisses a tenured teacher. As long as you don't do anything outstandingly bad, you’re likely going to just get a warning and some steps to take to fix the issue. Even if something goes in your personnel file, there are laws to prevent sharing most discipline information with anyone else, including future employers.

Finally, even if you do something the administration deems as egregiously bad, but you or the union disagrees—perhaps based on context or mitigating circumstances--the District is required to provide, at minimum, a notice of the charges against you, proof for those charges, and a chance for you to respond to the charges in a hearing before the board of education.
For tenured teachers who sometimes stay up late at night worrying about whether what they said or did in class is going to get them fired, it’s most likely not. You might get a stern talking-to, or a warning added to your personnel file, but your job is likely safe.