I would suggest an indirect route. Start with a gender therapist first and then you will have a professional to back up your words. In addition the therapist will be able to explain the need to your mother better. Yes you might be able to do informed consent but if you do so, you can't use the argument that you have explored other options. There could be therapy available through your school and if money is an issue, some therapist work on a sliding scale where what they charge is based on your income.