They are not supposed to be able to fire you, but if folks intend to they generally try to come up with some pretext for doing so.
First they often try to have you resign.
Second, if they think you won't or you don't they try to find something based on their alleged circumstances -- lack of funds to support x positions, last-in first out, no probationary teachers, not enough kids in grade 6, if they can.
Third, they try to base it on your circumstances -- failure to return a library book, absence, lack of control over classroom, poor performance, lack of respect for the principal, etc.
Very often a consultation with a lawyer who knows education laws or labor law can get you up to speed on the exact state of the law in your area and the lawyer may even send a letter that puts the school board on notice that you won't be a pushover.