In High Places
by Harry Turtledove

Description:
In the 21st-century Kingdom of Versailles, the roads are terrible and Paris is a dirty little town. Serfdom and slavery are both common, and no one thinks that's wrong. Why should they? Most people spend their lives doing backbreaking farm work anyway.

But teenaged Khadija, daughter of a prosperous family of Moorish business travellers, is unfazed. That’s because Khadija is really Annette Klein from 21st-century California, and her whole family are secret agents of Crosstime Traffic, trading for commodities to send back to our own timeline. Now it's time for Annette and her family to go home for the start of another school year, so they join a pack train bound for their home base in Marseilles, where the crosstime portal is hidden.

Then bandits attack while they're crossing the Pyrenees. Annette/Khadija is separated from her parents and knocked out, and wakes up to find herself a captive in a caravan of slaves being taken to the markets in the south. She's in a tight spot.

Then the really scary thing happens: her purchasers take her, along with other newly purchased slaves, to an unofficial crosstime portal…leaving open the question of whether Crosstime Traffic will ever be able to recover her!