WASHINGTON — Fourteen-year-old Brandi Levy was having that kind of day where she just wanted to scream. So she did, in a profanity-laced posting on Snapchat that has, improbably, ended up before the Supreme Court in the most significant case on student speech in more than 50 years.At issue is whether public schools can discipline students over something they say off-campus. The topic is especially meaningful in a time of remote learning because of the coronavirus pandemic and a rising awareness of the pernicious effects of …