Well, this is another one when I have to say, there is no right or wrong way. Since you are out, at school, it is only a matter of time until your Mother hears it. I would definitely not want her to hear it from anyone, but you.
You have to make up your mind when to tell her but I would make it as soon as possible. If she does not know, and I suspect she has an inkling, it will be one of shock and surprise and you will need to be prepared for that. However, it will also give you a chance to talk to her and explain how you feel, etc. Tell her what you told me. It is important to have her support.
Try to under stand her feelings, as well. I know from experience, that when I knew in my own heart, our son was gay, (he had not told us yet) my concern was not us, as a family, but one of worry for him and what he would have to go through, in life. Today it is much more open and accepted than when he came out. It will never be easy and there will be ups and downs, with your mom, but hang in there. It will all work out. You have to tell her sometime and now is better than later.