Well, this is kind of a grey area, to be honest.
Teachers that announce to the world that they're into pot are obviously a no-go, however teachers that smoke pot discreetly (whether it's habitual or occasional) seem to be a huge statistic amongst the USA's teaching population.
So, officially and on paper, no, you absolutely can not smoke weed and teach the children of your nation. Unofficially? If you can pass a piss test and your kids are passing their classes, no one's going to give much of a fuck what you do so long as you can make yourself seem wholesome. Fair warning from my own experience: male pot-smoking teachers get busted by administration way more often than female pot-smoking teachers.