Teach Your Kid To Guard His Own Belongings
There is no harm in making your child slightly independent, especially when she starts school. School is the first place where your child is going to be alone. This is the first time and the first place where she will be on her own, taking care of herself and her belongings all by herself. So, it can be slightly intimidating.
Talk to your child about why and how it is important to be vigilant about her things in the school. Tell her that she is now a big girl and so it is her responsibility to see that she comes back home with everything that belongs to her. Also, to help her out, label all her belongings and mark it so that it is easy for her to identify them. She is new to such an environment and so any sort of help from you is welcome.
Now, even after doing everything, it is quite possible that your child loses stuff at the school or any other place. Handle this with patience. Sit with her and try to recall where it must have gone. Yelling and blaming her will not sort out anything and she will be more scared to venture out with her things. Losing stuff by kids is way too normal. The only point here is how can you make your child responsible so that she is capable of taking care of her own things.