What episode on school rumble was it when Yakumo falls harima?

You're thinking of the iconic scene where Yakumo kisses Harima, not falls for him. This happens in Episode 11 of School Rumble, titled "Love, Pain, and Misunderstanding".

It's a pivotal moment in the series as it marks the first time Yakumo directly expresses her feelings for Harima, though he doesn't quite understand the situation at the time.