My 10 year old is lieing about homework to both teachers and us, what should we do?

She has been having problems for a while at PT conferences we discussed this and thought we had a plan. The teachers were to make sure she had it wrote in the school planner (but that lasted a wk) and her home room teacher was to have her put her homewrk in her mail box. She tells her homeroom teacher she has not homewrk in reading and the other cles, but they talked and now know what is going on. I do not think she has done homewrk for a month. I am going to ask if they could send her work home for the wk on Monday so we have it and know what day she does what. Any other Ideas, punishment is not working. She is Bi polar and she is in therapy and other treatments for this. The teachers just quit helping then they get mad at us. I can not make her do her homework when she does not have anything wrote in her planner (which we have to sign daily), or she does not bring home any home work ( and yes we look in every part of her backpack and any other thing in there.