Explaining bad grade on transcript
So I was told that you can explain bad grades on the additional information section of the common app. During my sophomore year, I got a B in a math class (which typically is my strongest subject) because I had a terrible teacher. This teacher is notorious at my school for her poor teaching, being lazy, not grading accurately and fairly, and being biased towards students. For instance, I received a C on a test that I should've got an A on because she misgraded, then she only gave me back half credit because I told her too late. And half the time I had to self study the material because she spent the class complaining about something off topic. Would this be a valid excuse for my poor grade or would this come off negatively and seem like me just shifting the blame.
You shouldn't write about it because these look like issues that the teacher personally has which is hard to quantify. If only one person got an A or something similar happened that you can quantify that would objectively show that your grade isn't as bad as it seems. I'd also note that a B your sophomore year won't kill your chances so long as you went on to perform well in harder math classes and/or you performed well on a a standardized test in the math section. Many people have one of "those teachers" and if you complain about them it won't reflect well on you.
Every school has a teacher like this. Literally, every single school has a teacher like this. For me, it was my freshman year Math 3 Honors teacher. Using additional information to complain about a teacher looks immature and petty. Also, a B isn't a poor grade. Obviously, don't let it become a trend, but many people who get into top colleges have Bs on their transcripts. As long as you do better on your higher-level math courses you should be fine (like @cp839 said).