Do highschool students in Washington, DC have to take certain exams at the end of each year?

It depends on whether your school is semestered or not.

So, if in the first half of the school year you have some classes, and in the second half you half different classes then you would probably have exams sometime around christmas, and then again at the end of the school year for each one of your subjects.

If you have the same 8 subjects throughout the whole year, then you will have the exams during the month before summer break.

If you have either of these systems, exams will consist of all the main material you have learned through out the time you have been doing that course, you will have exams for each and every course ( maybe not gym though, i dont know ) which is why it is always a good idea to keep your notes! Good Luck!