Son brought valuable item to school and another student took it home. Not sure how to proceed

I have a ten year old son, in 4th grade, and this past Monday he asked me if he could borrow my fitbit watch. I told him no because I was afraid he would lose it or leave it at school. He went ahead and snuck it out of the house that morning, and took it to school anyway.It took me until Thursday evening to notice it was gone, as I had a very busy and stressful week. When I noticed it was gone I immediately knew that he had taken it to school. He admitted it and said he gave it to one of his friends because she offered to take it home and charge it for him. My charger hadn't been working and I hadn't replaced it yet, so he said that's why he agreed.He said the little girl, who is in his class, has been making excuses all week as to why she has not brought the fitbit back, and since they did not have school on Friday I have not yet been able to get to the bottom of this issue.I am not sure how to proceed. It seems like the little girl either doesn't want to give it back, or maybe even lost it herself, or maybe she is just forgetting to bring it back. I don't know her so I have no idea what the case may be.So I am wondering if i should contact the school and ask them if perhaps they can contact the parents of the little girl and see if they can ensure she brings the fitbit back. I'm not sure if it is appropriate.I don't want to come off as a demanding or aggressive parent, and I certainly don't want to make the little girl or her parents uncomfortable, but the fitbit was an expensive luxury for my family that we would normally not be able to afford, and that is why I am so worried about getting it back. The cost isn't a drop in the bucket for us. My husband worked overtime at his job to purchase it for me as a surprise because he was very proud of me as I recently lost a lot of weight through diet and exercise and he wanted to get me something that would help me to continue my progress. He was so proud to be able to surprise me with it, I haven't even been able to bring myself to tell him that it's missingSo please, if anyone has any advice on how I can tactfully attempt to get my fitbit back without coming off as accusatory or aggressive, I would really really appreciate it. Also please let me know if you think it is inappropriate to ask the school to contact the other child's parents, and if so, what steps I should take instead. Thanks so much! via /r/Parenting http://ift.tt/2piQnk1