Please help with my 11 year old daughter?

Today was the second time I got a phone call from her school to say she hadnt turned up. My heart sank. I rang the school bus driver to confirm she got on the bus and got off at the school. She gets on in the morning with about 7 other children. They all get on a few doors up from me. From now on I will be walking with her to the bus stop to make sure she's on. She did this last year when she went to the town park with another girl who was 12 at the time and bad news. They went to the park and then to visit the girls sister in hospital and then to the girls aunts house. A few older teens who were in the park at the time between exams rang the school and reported the girls in uniform. Otherwise I may never have known where she was. We had a meeting in the school with the principal and the other childs parents. She was lucky not to get suspended and banned from seeing the other girl. I grounded her and came down really hard on her. Since then she has been brilliant. I even had a chat with her teacher last week who sang her praises and told me how clever bright and well mannered she is. I bought her a surprise as a reward. Im doing everything right as far as I can see but now this :(... I am going in to her school this afternoon to speak with her principal. Im so upset/disappointed. Im a single mum but her dad is actively involved in her life and has been since she was born. They have a close relationship. Im at my wits end. Has this ever happened to anyone elses child??? Please some words of advice.