File formal complaints... if they continue to ignore it, go find yourself a lawyer and sue the school.
When someone talks negatively about trans people around me, I go after them... I simply don't tolerate that type of behaviour. As an adult and professional, I DEMAND respect. If it is some random person saying a comment in every day life, I would gladly step up and see how well they can defend their opinions (if it goes beyond a random comment and moves into harassment? I would go to the police).
I actually had this happen twice in my classes this term. Two different teachers decided to tell stories of trans people... and ended up painting them in a pretty negative light. And, each time, after class they recieved a long letter from me how their behaviour was out of line and highly offensive. As the person paying for the classes I demanded the subject to be treated with respect. Both of them imediately opologized.
If they would have ignored me or said ANYTHING even remotely negative in reply .. I would have charge straight to the school administration and filed formal complaints. If the school would have ignored me, I would have run out and found a lawyer to begin legal action against them (such as creating a hostile environment, or actually since I am in california, would have used the trans discrimination laws to my advantage).
Like I said, you are an adult... you are paying for these schools... you and your child have a RIGHT for protection against this behaviour. If the school refuses to do their job, then it is time for you to take it to the next step and teach them that it isn't really an option... that they are messing with the wrong person.
If the kids continue to harass you outside of the school setting? Go call the police and charge them with harassment / intentional infliction of emotional distress.
If the police ignore it? Again, they don't have that option... if they do, they open themselves up to some serious legal action.
Show these people that you don't tolerate this.