From the west:
Take Route 128 to the Trapelo Road exit (the next exit south of Route 2). Head towards Belmont (a left at the end of the ramp). Follow for around 4 or 5 miles, almost into Belmont. This will take you past a light at a Mobil station (Lexington Street, Waltham), past Metropolitan Hospital. The Fernald School will be on your right.
From the city:
Go West to the end of Memorial Drive or Storrow Drive. Stay to the right, away from signs for Route 2, instead opting for Mt. Auburn Street. You'll pass a cemetery on your left; get into the right lane, preparing to take a right after the Star Market (which is about 1/2 mile away). You are now on Belmont Street. Follow for several miles into Belmont, going through 4 lights (counting flashing lights as well as "normal" ones). Just after the 4th light, bear right at the fork (Brigham's will now be on your left). Proceed straight; you'll pass Friendly's. Go straight for several more miles past another Star Market. Go straight; get into right lane and go straight through the light. The Fernald School will be on your left about a 1/4 mile up Trapelo.