When you're explaining any type of academic under-performance, be careful blaming other people.
Sometimes your academic performance suffers for a legitimate reason. Maybe you were ill and had to miss several weeks of school. Maybe you were only recently diagnosed with a learning disability. Maybe you had to help take care of your sister when your parents could no longer send her to daycare. You shouldn't hesitate to explain those circumstances that really were beyond your control.
But blaming other people sounds like this:
"I got a 'C' because of a personality conflict with the teacher."
"I didn't do well in Spanish, but the language department at my school is terrible."
"I was just 3 points away from 'A,' but my teacher refused to raise the grade."
When you make excuses like those that blame other people, the colleges inevitably think,
"Well, another kid in that class still got an 'A'."
I don't deny that those excuses may be legitimate in some cases. But sometimes, you get a bad teacher. It could happen in college, too. And after college, you might have a bad boss. Or a bad landlord. Or a bad mother-in-law. When that happens, you won't always be able to just resign yourself and blame someone else. Sometimes you're stuck and you have to figure out a way to make it work.
High school is a great training ground for this. If you really do have a personality conflict with a teacher, what are the other students doing differently that you are (or are not) doing? If the language department at your school really is terrible, what steps could you take to improve your own learning experience? And if you really were just 3 points away from an A, remind yourself that lots of things, from Olympic gold medals to sales awards at big companies are based on systems where the highest numbers win.
Colleges–and future employers–aren't looking for the students who blame other people. They want the students who find a way to make it work.