My Daughter Just Turned 7 And She Won't Stop Stealing.

My daughter just turned 7 and she won't stop stealing. She steals at school, grocery stores and at home. She steals stupid things like gum, erasers, pencils, candy bars and lip gloss. She will go as far as going though another students desk when they are doing activities. She will call the other student a liar saying the item she stole is her own. She lies and blames her stealing on her little sister at home. Please help me. I have tried every method from buying her extra things to grounding her. I even had her return an item she stole from the store. it's so embarrassing. I won't allow her to visit friends because I am afraid she will steal from them too.