How do I expose/stop subtle racism/clism in school pta?

I am not good at confrontation but have let this run on long enough. Two people on pta have been talking behind my back and subtly blackening my character. I dont have a problem with either of them so can only ume it is a racial thing or cl thing as i am black and they are white. I have tried for a year to be positive and encourage them. I heard them saying they have got rid of someone befor who they didnt like and they are all smiles to my face though. I was chair but backed down as they made it difficult. One or two others realise what they are doing but no one has clear evidence. Most of the members think they are just wonderful and indeed they are good at stuff just manipulative and sneaky to me. How should I deal with it? I enjoy doing events at school but feel if i cant sort it i need to get out all together as I feel I cant trust them. I thought about going to head but feel she is busy with running school and I dont want to bother her with this.