Could they really not find one single black, Hollywood-passable-for-a-teenager female? When the entire story revolves around a minor, why cast a woman in the role who looks older than almost everyone else in the episode, including the main characters? Being short and having their hair in pigtails does not enable someone who looks about 40 to pass for a 14-20(?) year old.
It would also have been much better if our heroes' roles had been reversed and it was Hutch who was working with the female detective. Open-minded, reasonable (apart from that moustache!) Hutch going from, "I don't want my partner replaced by someone who's only here because they tick the necessary minority boxes," to actually respecting and giving in to his attraction to Inspector Meredith would have meant something. Starsky, on the other hand... at this point I wouldn't leave him unsupervised with Lassie. So the fact that he almost instantly switches from being antagonistic towards Meredith to wanting to bang her brains out really isn't the deep message of racial acceptance that the writers probably wanted to get across.