If there's a case that my teacher (or teacher's assistent) is somewhere near my age or we have known each other for a while (usually educators want to be as quick as possible to befriend students during class, so that would be approximately two weeks), should I address them in Tu or Vous? Another case is that what if I've chatted with a person whom I've just met for an hour and after a while I text that person in Tu, would I sound repelling? Merci beaucoup in advance!