So, for the people that's trying to get the Brigitte storyline. First do all the homework that you have for Torbjörn class, that should start it.
For the Mei storyline. It should start similarly, do all the homework for her class.
It's important that you do your homework as soon as you can, since both storylines start the only the next class that you got it (I think).
You can do your homework on your computer, just answer some simple questions, if you get any wrong answers your caracter will just delay the question for later.