Teenages acting out

Why do some teenage act out, first I feel like I need to do this blog because I have a teenage grandchild who is acting out special in school and also at home. My son have two girls one is 16 years old and the other is 13 years old the one who is 16 years old is an A, 4.0 student and she is also a cheerleader she is a very good child she do not cause any problems at all. In the other hand the other one who is 13 years old is a bad problem, she was making good grade in school an A.40 to an D's and f's student she was playing basketball she got kick off the team. she just got suspended from school she say bad things to her mother but not her father which is my son. Her mother and father is not together anymore first I through that was the reason why she was acting out like this but we all found out that she is having sex and we all know that can cause you to have behavior problems. She come home with spotes on her neck I also had a talk with her to tell her about when I was a teenage I have made a mistake of loving someone and how I may a big mistake I got a baby and I do not won't that for her. I can remember when I was a teenage I through I knew everything know one could not tell me anything.I won,t to ask you a question why do some parents alway saying that just their hormones, and why when you are a teenages you take so many risk. I know I did when I was A teenage I have four children they was petty good children they all finsh school and went to college thank god for that. I thank that kids can get around other kids can cause a problems if that kid have a bad behavior. My son told me the talk I had with my grand children it did no good she kids have to learn the hard way. I am going to keep praying for her holping she will grow out of this behavior.