In the S5 Episode "Tough Love" Buffy tells Dawn if she won't go to school Buffy will be found unfit as her guardian and they'll take her away.

In the S6 episode "Gone" Buffy is turned invisible by The Trio's invisibility ray. She goes to Social Services and makes the social worker think she's going crazy, and replaces Dawn's files with pages that only say "All work and no play make Doris a dull girl".

"They'll Take You Away!"

Buffy's Kitchen

Monday Morning

10:45 a.m.

The phone rang and Willow answered it.

"Hello. Yes she it. Just a minute."

Willow put her hand over the mouth piece and said, "Buffy it's for you. Some woman."

Buffy took the phone.

"Hello. This is Buffy …. Oh, hello …. What? What did you say? …. And you LET them? …. How could you let them do that? …. Where did they take her? …. Oh, you can bet your sweet ass I will!"

Buffy slammed the phone down so hard, she broke it.

"Buffy? What?"

"They took Dawn. They went to the school and took her right out of class. They took her away!"

"Who? Who took her? And where?"

"Some social workers. They took her downtown."

"But how could they do that?"

"That was Dawn's principal on the phone. She said they had a court order to take custody of her."

"But I thought that was all taken care of when you were all invisible and made it look like the social worker lady was crazy."

"I thought it was too, but apparently somebody reopened the case."

"What are you going to do?"

"I'm going down to the Social Services office and find out just what the hell's going on. And if I have to, I'll drag Dawn out of there, and DARE anybody to stop me!"

"Uh, Buffy? Before you go – you might want to cool down some. I think the last thing you want to do is piss off a bunch of red-tape civil servants. Sometimes they DO have the law on their side."

"I've got to get dressed. I'll let you know what happened when I, when WE, get back."

"Just – don't hurt anybody. 'kay?"

To be continued