I took the SAT Math II subject test (just graduated 10th grade) as an assessment of my skills to see how much I knew. Would it be possible for me to take it again next year and only have that score count?
Almost all colleges allow the applicant to choose which Subject Tests to send, but there are a few that require all scores. You can read through the following thread, but confirm with your target colleges.
http://talk.qa.collegeconfidential.com/sat-act-tests-test-preparation/1912440-must-you-submit-all-sat-act-or-sat-subject-test-scores-p1.html
Alright, thank you. Collegeboard also gave me a “Range” in which I could score if I retake the test, does this mean that even if I score above the range in a year, that won’t count?
No. Your score will count.