This is killing me from inside

Yesterday I was talking to my younger sister and between our conversation she mentioned how some guys stalk her and send dms online.The most disturbing was that a guy was constantly staring at her in the bus when she was comming back from school. She mentioned that the guy even did some unusual action/gesture toward her. Now this thing is scaring me from last night, I live in hostel, way far from her and now I am constantly worried about her safety. she is still a kid. So please suggest some measures she can take.