1° Do not try to convert you Korean grades into American grades.
That’s where context comes in: if half the class got a 50 or lower, than 50 would be a B- or so in the US, right? It wouldn’t be converted as an F.
2° Apply to colleges that will value your high SAT score and count 1st quarter grades in order to offset your current ones. Private holistic admissions and good regional schools are a good bet.
Elon would be a good one. UNC Wilmington and NCSU would be other good ones for instance.
What do you want to major in?