Stained uniform

I’m not sure what they do at my daughters preschool but I’m tired of her coming home with stained clothes. The worst thing is, even after wash it doesn’t come off. Hasn’t even been two mnths since she started pre school and majority of her uniforms are stained. They’ve also misplaced one of her t shirts. Does anyone know I can take this stain off? Please see first comment for the image of the stain.