My daughter started that a few months ago and really pulled a big one on me. Had me believing her first grade teacher was the meanest horrible person in the whole world. I called her teacher up and made a arse of out myself. I finally caught on. This is what I did.
She came home from school and we talked about it, she didn't want to budge so she had to sit on the couch until she wanted to tell me the truth, she was on the couch til bedtime yes that long and still wouldn't tell me the truth so when she came home the next day from school she went straight back to the couch. She finally gave up and told me the truth. I talked and talked with her and told her she can always talk to me, if she has done something wrong ok tell me about it but if you lie your in alot more trouble. I stressed to her it's 10x worse if she lies. So far she hasn't lied again. I explained also how she lost trust with me.
I also double check everything now, if she tells me something I will call her teacher, ask the kids outside, ask the neighbors etc. I always get both sides of the story now and no longer take my daughter's word she lost trust with me I can't trust her.
(my daughter has to EARN my trust back and that doesn't happen overnight) her lies were straight out embarassing and could have gotten her teacher in trouble