Storm Portal by Michael R. Stern

Time Travel is real!!

History teacher Fritz Russell walks through his classroom door to find Robert E. Lee resting in the woods on the day after the surrender at Appomattox. Fritz loves history, and entering the past seems a gift, the chance for a great adventure. He’s been bored teaching the same old stuff year after year, and here’s a chance to make it new. But when the classroom door opens to the Oval Office, he realizes that his time portal might also be put to serious purpose. The president wants his help on behalf of national security. Fritz does not believe he will be endangered by using his door to travel across time and space. And the president appears to be a great guy. But will Fritz’s own government consider him expendable if he cannot solve the mystery of the portal? He doesn’t have much time.