how do i make my parents understand the real side behind me?

well, my older sister and mother always tell me that im stupid and that i sneaky. they think i hang around with some guy at school and do things with him, which i dont. What i dont understand is why dont they beleive my side of the story. they always jump to conclusions and asume im doing something secretly. just because im a bit of a shy person doesnt mean this right?
any help on how i can make them understand, and how to gain trust again?