Still they didn't tell Buffy of the plans to resurrect the master. Well it isn't as if she could do much about it. Jenny is still alive, then Angel didn't loose his soul yet.. (I didn't think of the time line).
Buffy saw too much in the past when she was the slayer in order to forget it (like she said she didn't feel to belong to the "cheer leading crowd")

Author's Response: Yes, this is way before Angelus would have popped up.