Most colleges use a holistic review process, meaning that they look at grades in the context of the student’s school and along with other information in the student's application. To find out if a college uses holistic review, look on their website or ask someone in the admission office.
Admission officers that review holistically will look at the student’s GPA year over year to see if it remained the same through high school or improved.
They'll also look at the student’s SAT or ACT scores, extracurricular activities, letters of recommendation, essay, and any other requested items to get a full view of the student. Different colleges weight these factors differently, but if a college asks for something on the application, you can be sure it’ll be used in the admission decision.