Cavegirl (1985)

On a class excursion to a cave with stone age paintings the clumsy Rex gets lost. A mysterious crystal opens a gateway in time and sets him back to the stone ages, where he meets a group of prehistoric fellows and the gorgeous Eba. While teaching her English and doing some pseudo-research on her fellows, his main goal is to get her sleeping with him. However the curiosity of the clan chief disturbs his attempts. imdb