Otome wa Boku ni Koishiteru

Which one is the boy again? Mizuho is the one in the lower right corner.

Mizuho Miyanokouji is a nice guy from a rich family. When he hears that his grandfather left behind a request in his will, he's happy to oblige. Though he becomes a little less enthusiastic when he finds out that that said request is for him to disguise himself as a girl and attend a specific private girls' school. Don't worry, it's all explained later...

Now he has to deal with keeping his gender a secret, helping the girls who admire "her", and learning how to be a better person.

Based on the shoujo-ai visual novel by Caramel Box, although it is more closely related to the Otokonoko Genre than the Yuri Genre.

Compare and contrast Maria Holic. Incidentally, both series have a character called Mariya.

Now has a sequel, Otome wa Boku ni Koishiteru ~Futari no Elder~, Was released on June 30, 2010, which will be receiving an anime version itself.

Tropes used in Otome wa Boku ni Koishiteru include:

The Antagonist - Takako acts as one in several plots, such as during the swim challenge and the ribbon issue.
Baker's Dozen
Bishie Sparkle - Several times, mainly for Takako and Sion. Mariya once gets rainbows as well.
Bishonen - Mizuho is very girly even when not crossdressing.
- Something of a side effect of the original artists drawing style; guys and girls look uncannily similar, leaning more towards the girls side.
Blatant Lies - "I am perfectly calm!"
Bleached Underpants
Bridal Carry - Mizuho uses one of these to carry a sick Shion from the auditorium to the nurse's office.
Cat Smile - Kana often has one during her Chibi-mode.
Cooking Duel - The swim competition beteen Takako and Mariya.
Crash Into Hello
Cute Ghost Girl - Ichiko, the hyperactive bisexual ghost who was in love with Mizuho's late mother.
Defeat Means Friendship - While Takako was never overly hostile towards Mizuho, once she loses the Elder vote to Mizuho (and seeing Shion and Mizuho's actions afterwards), she becomes friendlier.
Disappears Into Light - Subverted When Ichiko disappears after "marrying" Mizuho, only to reappear back at the dorms later in the episode.
Dropped a Bridget On Him - Usually the invariable result of Mizuho-chan fans who watch the show after having "fantasies" about "her", and when Takako finds out about Mizuho-chan's real gender
Even the Girls Want Her - For a given value of 'her', anyway.
Flung Clothing - Takako and Mariya fling their clothes when challenging each other for a swim competition.
Gratuitous English - Anime opening.
Hair Antennae - Mizuho
Hot Shounen Mom - Mizuho-chan's Missing Mom. Mizuho-chan looks exactly like her, in spite of being her son.
Ill Girl - Sion
- Ichiko used to be one. She didn't get better.
It Makes Sense in Context The whole idea of the plot.
Late for School - Lampshaded in Ichiko's flashback. She even calls it cliché.
Motor Mouth - Ichiko ("Oneesamaoneesamaoneesamaoneesamaoneesama....")
No Periods, Period - Averted: Mizuho, despite being male, claims to have one to get out of PE class.
- Shion commiserated and spent several moments going into how hers have rather heavy flow too, before remembering that Mizuho isn't a real girl. He's just THAT good at faking it.
The Ojou - Apparently everyone.
Onee-Sama
Puni Plush - In the manga. It makes Mizuho's assertions of manhood less credible than they already are.
Rapunzel Hair - Mizuho always had long hair even back in his childhood. It sure helped with the disguise too.
Romantic Two-Girl Friendship
Schoolgirl Lesbians - One-sided. There's also a pair in the game, but this is only subtly hinted in the anime.
School Play
Sins Of Our Mothers - Mizuho lives in the room that his late grandfather arranged for him - which is the very same room that belonged to his Missing Mom during her school days and it is also the room where Mrs. Miyanokuji's devoted Ill Girl kohai, Ichiko, died of exhaustion while waiting for her to return. Things ain't that grim, however, as it turns out that said kohai survived as a (rather cheerful) ghost and it is Mizuho's task to make amends to her for his mom's untimely departure 22 years ago.
Skinship Grope - Done to Mizuho surprisingly enough.
Stupid Sexy Flanders - Mizuho is waaaaaay too pretty.
Super-Deformed - A lot of the girls go into chibi-mode on a regular basis. This is their default form in episode 13.
Tsundere - Takako (and ~~lampshaded~~ exaggerated in the OVA, with her role as Tsunderella in an hilarious reinterpretation of Cinderella).
Token Loli - Kana
Twelfth Night Adventure - One of the sweetest and most heartwarming examples of this genre.
Unlucky Childhood Friend - Mariya.The opposite is also possible in the game, of course.
Verbal Tic - Kana's "Nanodesuyo"
What Do You Mean It's Not Awesome? - Cheesy violin music and closeups during practice for a certain scene.
White Prince - Mizuho, before starting to crossdress and pose as an Ojou
- Even afterwards, some of the girls talk about his more impressive moments as him acting like a fairy-tale prince. Even the ones that don't know he's a guy.
Wholesome Crossdresser - (foremost character, above)
Why'd it have to be ghosts? Takako's voice breaks and her body shakes as she denies ghosts existing. Yukari is scared of them too, but warms up to Ichiko after spending some time with her.

Episode 13, the OVA, provides examples of:

Calling Your Attacks - In the battle between the fairy godmother and the evil queen.
Dramatic Thunder - Lighting strikes in the beginning of Tsunderella. Since she has the high ground, she's also struck.
Drop the Washtub - Occurs more than Takako is comfortable with.
Fractured Fairy Tale - The setting:
- Cinderella - Takako is Tsunderella, with Marija, Ichiko, and Yukari as the evil step-sisters. Also the main plot.
- Fairy Godmother - Sion.
- Gingerbread House
- Little Red Riding Hood - Michiko.
- Snow White - Kimie.
Genre Savvy - Tsunderella seems to know about the fairy tales she's put in.
Happily Ever After - The story ends with the usual Happily Ever After ending with the prince and Tsunderella together. Then it's subverted when they don't live prince's life, but Tsunderella's, and have to work for the evil step-sisters. And finally double subverted, since they're actually happy anyway.
I Have the High Ground - Takako in the opening.
Love At First Sight - The prince and Tsunderella, complete with discussion.
Leprechaun - Sion is dressed like one.
Magic Mirror - Kei asks her mirror who the most evil one is. She's happy to hear she is.
Naughty Tentacles - Said, and shown with a heavily pixelated image, to happen to Tatako, should she break the classic midnight time limit.
Noblewoman's Laugh - Takako laughs like this in the beginning of the story. Just after she wonders why she was laughing so much, in a few scenes showing her as extremely accomplished. Then she laughs some more. And is struck by a falling washtub, which causes her to fall off the cliff. Then she's struck by lightning again.
Nosebleed - Takako's nose bleeds when looking at the prince.
The Nudifier - Sion's first attempt at creating a ball gown for Takako fails. She only manages to remove both of their clothes, and turn them normal-proportioned.
Prehensile Hair - Sion caputures Takako with her hair when she tries to run away.
Twelve-Episode Anime: Including an OVA.
Stat-O-Vision - Kimie's glasses apparently has this function.
X-Ray Sparks - Happens to people struck by lightning, mostly Takako.