@lemming Make your profile private, and remove all the damming evidence you can. Now if one of your old friends feels the need to mention something to an employer, unless they have proof then you can deny it as hearsay. If you are fully “outed” then admit that you smoked when you were younger, that you don’t now, and that this in no way interferes with your ability to teach.
I don’t think it will come up unless someone is posting photos of you smoking on facebook, which isn’t allowed. And I don’t think they will be that concerned with you unless you start doing drugs during school hours, or having sex with the kids. Be cautious (not paranoid), and if all else fails, move to California.