You’ve never met anypony by that name, at school or otherwise. You’re also eight and have never seen The Brady Bunch. Why even ask this right now?

You decide to just tell the truth, or at least some of it. The pair seem less interested in this than before. Less because they don’t believe you, and more because they don’t care.

Diamond Tiara rolls her eyes when you finish, “If some smudged-up, old mirror is so important to you, why don’t you just knock?”
“Yeah, they’re giving tours of the castle today,” Silver Spoon adds. “Of course, we were on the first tour.”
“And would you look at the time? They might have finished for the day.”
“Wouldn’t that be a shame if Little Liza had to wait until next Tuesday?”

They both grin at each other. They’re right though! Princess Twilight was supposed to start giving tours of the castle every Tuesday! But with it being this late, they’re probably done for the day. You can’t wait a week! If they’ve been on a tour of the castle, they could probably help you navigate. This would make their help even more valuable if you sneak in. That is, if you can figure out how to convince them.

On the other hoof, you could just try knocking. Or maybe somepony on good terms with Princess Twilight could help you get in. Do you even know anypony like that?