Teenage Mood swings. This behaviour?

Hi, My daughter is 14 years old. She is good in studies , behaves okay when in good mood, plays with her sister, goes to music classes and listens to music etc. However she is always closeted in her room, locks the door, very secretive in work, browses incognito, gets tensed when her marks are slightly lesser than expected etc. She becomes violent if we take away her tablet. She sees videos of a particular teenage gang and seems to be obsessed with the same. Certain times she becomes very violent with her sister even for small issues. SHe tends to be aloof even if when visitors are there in the home OR even when we go visitng relatives or friends. She is very reluctant to come with family to any place and very difficult to persuade. What do we do to cope with this behaviour?