@roobs -
This is gonna be long; apologies in advance, maybe spoilers.
You will find almost no explicit yuri in this series. You'll find a horribly lonely girl obsessed with getting a boyfriend and unable to make friends who manages, unexpectedly, to make a complex and intriguing and sexually charged set of female friendships. It becomes clear that Tomoko likes guys but has no particular need of them. What she needs is other girls.
The first yuri plot strand has to do with Tomoko's best friend Yuu, a sweet, dumb sex bomb she's known since middle school. Tomoko drools after her. But Yuu is sexually involved with guys and is out of Tomoko's reach, except for persistent harassment Yuu tolerates for the sake of friendship. Later, Tomoko accidentally mind breaks a girl named Uchi on a school trip and Uchi begins a strange, ongoing yandere-like crush on her "disgusting" object of affection. Every now and then, there are startling hilarious gags based on Tomoko's obvious partial inclination toward other girls.
The "yuri" doesn't originate from sexual attraction per se, but from Tomoko's growing receptivity to other people. What saves her life, perhaps literally, is an angelic senpai named Imae Megumi, introduced at ch. 21, who cares for her and watches over her. Megumi is a subliminal and later a conscious pattern for Tomoko to follow. Tomoko doesn't realize how much Megumi means to her until the emotionally shattering ch. 115 and 115.5, when it finally dawns on her too late. But as Tomoko's new friendships evolve, Megumi quietly cares for her.
Except for the hilariously on-the-face-of-it ch. 51, "I'll Swing Both Ways", featuring Tomoko's edgy friend Hina and Megumi, the "yuri" phase of the series begins with a field trip to Kyoto, ch. 71-80. This is where she meets most of the girls who will become her friends. From then on, friendships form and deepen and complicate, with jealousy and competition and sexual confusion thrown in.
Where it really starts is the Disneyland field trip sequence ch. 125-131, where Tomoko inexplicably becomes the general center of her friends' interest. There, other than Uchi, the three main competitors for Tomoko's favor ramp it up: Yuri, Hina, and Asuka. Tomoko really has a problem with Asuka, who she'd say "yes" to before she knew her lips were moving.
Don't expect yuri outside of friendships. It's not what you'd call subtext; it's stronger than that. What you have is the story of an insanely miserable misfit, probably on suicide road, redeemed through friendship when she ironically winds up popular among a fun group of girls. Who may have a thing for her.