For BB xM Rev C the expansion pin 24 (GPIO168) is reserved for the I2C SCL signal and it cannot be changed without tweaking U-Boot and kernel settings. You can export GPIO168 just fine and it appears as if you could change the state, however the pin stays high no matter what you do.
Instead one should use pin 22 for instance (GPIO157). This works fine.
echo "157" > export
echo "low" > direction (voltage on pin 22 should go low)
echo "high" > direction (voltage on pin 22 should go to 1.82V)