I wouldn't worry about it... you say you're active, with all those sit ups and stuff. My brother is you're height and weighs less than you. Some people just have a naturally low body weight. While gaining some weight might be good, it shouldn't be a huge stressor in your life. Oh, and those tests they do in gym class aren't always the most accurate anyway... last year, we did about 6 different tests in my class, and most of them were fairly accurate, but the waist-to-hip ratio test said I was obese... I'm 5'8 and weigh 115 lbs. I just happen to have a teeny tiny waist and slightly larger hips.
So don't trust that every test is completely accurate.
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If you hadn't used poor grammar, I wouldn't have needed to correct it.
I am not at fault.