I am a parent. What should happen if I tell the school that I am Lesbian, Gay, or Bisexual?

Basically whatever you want, in that if you want that kept confidential it should be. If you are informing them that you have a gender certificate then it is illegal for them to divulge that to any one.
You may be telling them in order for them to be aware that your children could be at risk of being homophobically/transphobically abused if they don’t make sure they are inclusive.
What is important is that the school is a safe space for every child, and that you feel comfortable and able to participate effectively as a parent.