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.
2 answers
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).