Mountain Brook (32-3) will return to Birmingham to defend its 2013 state title. The Spartans will play the Carver-Central Phenix City winner Wednesday at 1:30 p.m.
Grissom finished 23-12 despite 16 points from Reed McClelland and 11 from Devorice Milbry.
The critical stretch came near the two-minute mark of the fourth quarter. With Mountain Brook defending a 48-44 lead, Grissom got two defensive stops but committed immediate turnovers before advancing the ball.
Shearer made it count, hitting a 3-pointer from the corner to make it 51-44 with 1:51 to play.
Grissom committed 17 turnovers, and Mountain Brook outscored the Tigers 24-8 off turnovers.