Carlmont High School is at 1400 Alameda de las Pulgas, Belmont, CA 94002

From 280, take Highway 92 East (Toward San Mateo). Exit on Ralston, and turn left (toward the bay) (this is non-intuitive, but the exit from Hwy 92 East does a complete 180-degree turn before intersecting Ralston, so a left turn from the off-ramp onto Ralston heads you in the correct direction)
Turn Right on Alameda de Las Pulgas.

From 101, take Ralston west.  Turn Left on Alameda de Las Pulgas.

Note that the map of the high school is oriented very nicely for the school, but that north is pretty much at the bottom of the map.