Help me understand Dumbledore's actions....

[staffordbly]

No: HPFGUIDX 141676

There's a couple of things that don't add up to me in HBP :

1) Why didn't Dumbledore boot Draco from school after poisoning two students? Why did 
he allow him to stay and eventually find a way to let DE into the school and lead to 
Dumbledore's death?

If, as Dumbledore says, he KNEW what Draco was up to, why allow him to get that far?
If, as Dumbledore says, he also knew that Draco could face death from Lord V if he failed 
in his mission, than bootng him from school would of given him a perfect excuse for why 
he couldn't carry it out.....or even if it DID lead to Draco's death why spare him and allow 
two students to almost get killed, DE getting into Hogwarth's and Dumbledore's own 
demise? Don't make sense.

2) Why did Dumbledore immobile Harry when they raced back to Hogwarth's at the end? If 
he knew it was Draco, as he implies, certainly he and Harry could of taken care of 
him...and even had a fair chance at handling the unknown amount of DE's, since Phoenix 
memebers were on hand...plus Snape would help.

HELP!
Staffordbly