Answer: It depends on your school. In some places they have Night School or another "alternative education" option where you can retake required classes that you previously failed. Or you may have to give up a study hall or an elective class to retake the class during the day. Or try gain next summer. Worst case scenario: don't graduate on time and spend a 5th year in high school.
Explanation: Do your best now to pass the class. Talk to the teacher who is supervising the online classes to see if there is a way to retake the tests you have failed, or turn in missed assignments. Talk to your counselor or dean to find out what your school's policy is for making up failed classes.