What a hoot!  So Big D's got a fantasy life.  I love it, especially:  

"I'm on the Jared Subway diet."

And

"Don't worry, Barbie, there's plenty of Big D to go around."

Oh, this is pure delight  (...and I usually hate smarmy Marauder tales.)

I especially like these images:

He would ask her out time after time, and she would turn him down. Sometimes the question would just pop up out of nowhere, and sometimes he would work up this big scene to catch her full attention.

Her mind screamed “NO!” by default, but her heart took a slow, burning dip into her stomach at the mere thought of him.

And the ending is perfect: shows an impish side to  the rather staid Head Girl.

What a romp. Can't say who I like better the Dreadfuls or the Playboys!

 Certainly the Hat's reaction is right up there: "I feel strong emotions at play.  Anger?  Frustration?  A need to obliterate someone near you?"

Oh, darn, I thought he was going to 'have' Lucius.

But love Prof. Cabbageata and the idea of Sprout and Filch in the broom closet.

Such an inriguing beginning!  I can't decide whether Rowena is a vengeful, pouting child, a visionary rebel, or a far-seeing sage.  but that's what good stories are made of--doubts and unrest and strife.  I like your way of expressing yourself.  you seem to be pushing the boundaries of word meanings, but that can work to transport the reader to a new place and time.

I hope to read more of this.

Indeed, your sketch has intrigued me.  I love to hear more about the unexplained arcane devices with which JKR has casually endowed her characters.  You have brought the clocks to a kind of startling life and usefulness in the Headmaster's hidden chamber, and your analysis of Severus Snape's relationship with his two Masters is equally thought-provoking. 

Snippets that gave the piece real life for me:

--the shape of the room and its meager contents--except for the clocks, of course,

--the differences among the clocks,

 --the untouched lemon drops and their reason for being there,

--the word perii on the trunk,

Thanks for a delightful read!