I like the above posts by Pavarti & Kimberly.

Dumbledore saw some goodness in Draco, something worth saving.
He also had Harry to protect, and if he (Dumbly) was killed when
Snape wasn't there, Harry would have come out swinging.
So, Dumbledore used his occlumancy (spell?) to tell Snape what to do.
This also saved Snape's cover, once and for all.

Surely Snape knew of Harry's presence.
He also had every opportunity during the run out to the gate,
but he just kept on teaching.