LOWELL – Pinned inside its own 30-yard line for much of the first half of last night’s Div. 1 North battle with Andover, St. John’s Prep first relied on its defense to maintain order, then turned to junior running back Johnathan Thomas in the second half to run away from Andover, 21-0, at Cawley Stadium.Limited to 24 first-half yards, Thomas exploded for 183 yards and touchdown runs of one, 19 and 48 yards after the break as the Eagles’ offensive line of Sean Lovett, Domenic Hooven, James Fagan, Kent Blaeser and Sean Hoey carved gaping holes in the Andover defense. Thomas finished with 207 yards on 27 carries.Andover (10-2) certainly had ample opportunity to take early control of the game but its four first-half possessions ended on downs at the Prep 22, on a missed 29-yard field goal attempt, on downs at the Prep five, and on Conor Powers’ interception at his own 15.When the Golden Warriors missed another field goal from 31 yards to start the second half, the Prep got down to business.Remarkably, Thomas had the ball in his hands on 20 of the Prep’s first 22 plays in the second half as the Eagles ripped off scoring drives of 80 yards (10 plays), 90 yards (nine plays) and 50 yards (two plays).Thomas’s one-yard run broke a scoreless tie with 3:31 left in the third quarter and Stephen Cammarro’s first of three PATs made it 7-0.Andover – which had little trouble moving the football and finished with 309 total yards – countered quickly but saw yet another promising drive end at the SJP 10 when Alex Moore stripped and recovered the football following Will Heikkenen’s 23-yard reception from quarterback C.J. Scarpa.Eight Thomas carries and one reception later, the Prep had a 14-0 lead thanks to the junior’s 19-yard blast up the middle and Cammarro’s kick with 7:12 to play.Two plays after Andover failed to convert a fourth down, Thomas blasted through two tacklers at the point of attack and sprinted 48 yards for the putaway score with 4:48 remaining.St. John’s (10-1) faces Brockton Saturday night at Bentley College for the Division 1 championship.