Revenge of the Black Knight - The Time Travel Chronicles, #2

Time travel rule no.1 - No romance! After their Ancient Egyptian trip, 16-year-old Madison and Riley vowed never to time travel again. They'd set the world to rights-just-so that was that. Only back at school, not everything is right. A fellow student is missing. And he's not off sick or on holiday. No-one but Maddy and Riley even remembers his name. To get him back, they must travel to medieval England and repair his family timeline.
They plan to enter the period, fix the problem and leave-quick and clean. But, as medieval intrigue swirls about them-with murders, a castle siege and a mysterious knight in black bent on revenge-new loyalties draw them into a web of danger. Weapons and armies can change the course of history but Maddy knows matters of the heart can warp the timeline too. She avoids flirty knights and nobles, but a relationship based on mutual antagonism seems safe enough-until the hostility ebbs away and a more dangerous emotion creeps in.
They meant to tread lightly through time. Instead, they're leaving giant yeti-like footprints behind them, corrupting the time continuum and risking the future with those they love. A high-stakes fast-paced time travel adventure in medieval England.