My GD has her coat “stolen” from school last winter. Despite various searches it never turned up and we kept an eye out in case we saw another child wearing it. It turned up in the summer term at the back of a cupboard where someone had shoved it, whether that was another child or maybe a cleaner or whoever we have no idea but she did get it back 2 terms later! Without any evidence you can’t really be certain the shorts were stolen, maybe they fell out of her bag, maybe another child hid them as a “game “, who knows. They might turn up. PE teachers usually have regular kit checks, so it’s unlikely another year 7 would have stolen them to wear them surely? You could embroider her name somewhere on the shorts or mark them with permanent marker pen on the label.