Michael R. Stern 
Storm Portal [EPUB ebook] 

When history teacher Fritz Russell walks through his classroom door, he finds himself in the company of General Robert E. Lee – over 150 years in the past.
Fritz finds his sudden time trip to the past both a gift and a chance for great adventure. But when a portal opens to the Oval Office, he realizes that the mysterious gate could also be put to a more serious purpose.
When the president hires him to help with national security, Fritz doesn’t believe there is any danger in traveling across time and space. But will his own government consider him expendable if he cannot solve the mystery of the portal?