how do I get people at my school to use my preferred name?

Just tell them you prefer to go by Blair....where a name tag if that helps.
Or get you a jacket with the name stitched on there to remind them.

Sometimes people forget and it's not intentional.

For instance when I first starting going out as a female I went by Fran and used that name for about 2 years, later changed it to Megan. There are about 2 people when they see me still call me Fran, although that change happen 7 years ago.