The Last Timekeepers and the Arch of Atlantis

When 13-year-old Amanda Sault and her annoying classmates are caught in a food fight at school, they're given a choice: suspension or yard duty. The decision is a no-brainer. Their two-week crash course in landscaping leads to the discovery of a weathered stone arch in the overgrown back yard. The arch isn't a forgotten lawn ornament but an ancient time portal from the lost continent of Atlantis.
Chosen by an Atlantean Magus to be Timekeepers--legendary time travelers sworn to keep history safe from the evil Belial--Amanda and her classmates are sent on an adventure of a lifetime. Can they find the young Robin Hood and his merry band of teens? If they don't, history itself may be turned upside down.