Will I be able to pass Basic Training if I'm 125lbs at 6"1'. I heard about the requirements and I'm kinda scared about that. (I'm a sophmore in high school)
My suggestion: follow this of your family and doctor agree. My son has used this plan with a friend in your similar build and it's been amazing! You have to gradually ramp up what you eat to gain muscle like this, so do your research and bulk up!
Underweight by a little isn't a big issue. They'll give you supplements to add to your diet.
That said, you want to be more toward 155, if you are 6"1'...I was about 130 in basic, and had the cadre yelling at me to eat more.
I had a cadet with <2% body fat and he had the pleasure of drinking at least 3 Ensures per day during BCT. He weighs now, at age 27, within 5 pounds of his weight when he entered USAFA.
There are a few ppl who do put on weight during BCT (mostly muscle?). Being underweight at your height as a high school soph is not unusual. My 6'4" son weighed all of 118 on his 16th bd (at 6'2"). He tips the scales at 190# now.