Yui and Mio have some in early episodes. There was the time Yui became entranced when examining Mio's hands. There was the time they were in a hot spring at training camp and Yui clasped Mio's hands and thanked her earnestly, eliciting a shy reaction. And there was the time Yui praised Mio's lyrics, again while clasping her hands, and again getting a similar reaction. (Also like last time: Mugi's reaction)
Rather subtle, but Mio sometimes seems to have a thing for pretty ladies.
Yamanaka-sensei also at times seems ... rather interested in the girls. Especially Mio.
More like she likes to force them into cosplay. They end up liking it (even Mio).
And Mugi, in turn, seems to like Yamanaka-sensei quite a bit.
It's not specifically Sawako; she just likes when "two girls bond together" (in her own words). Also, she doesn't disagree with girl's love as long as both "express interest".
After Azusa joins the band, Yui seems to become extremely fond of her and hugs her a lot. It's an affectionate bunch, all right.
Yui is just like that, no sexual implications here... until ep. 12, when she tries to kiss Azusa.
Don't forget that when Azusa looks at their past performance in chapter 10 of volume 2; she even gets a Nosebleed from Mio's Panty Shot
The girls also seem to get sensual pleasure from Mio and Azusa's moe responses, so they often try to provoke them.
Concerning those two. For Valentine's day, Azusa thought about giving Mio some chocolate before giving a Suspiciously Specific Denial to Ui. Note that in Japan, Valentine's Day is a day reserved for girls to give chocolates to the boys they like.
Ui and Yui may also take their admiration for each other a bit far, especially in the Christmas Episode. Since it's between sisters it is likely innocent, but that won't stop the yuri fan boys/girls.
U&I which is a song for Ui sang by Yui. The lyrics could easy fit a lover instead of a sister.
In the manga, the student council president turns out to be very smitten with Mio.
Hopefully you're used to it by now because the Les Yay gets maxed out as far as it can go without a wedding in the Romeo and Juliet chapter of the manga (Or rather, the Romio and Jurietsu chapter....it's a pun, guess who the leading roles are) and the episode of the anime that follows. Complete with personality reversal.
Then there's the chapter where Azusa and Mugi are alone in the clubroom which just dripped with UST well... some kind of tension.
The time when Mugi and Ritsu hit the town with just the two of them has "DATE" written all over it.
The episode in which Ritsu became jealous of Nodoka for spending so much time with Mio. Almost feel like a girlfriend being jealous to a boyfriend. And to say nothing at the end where the two made up when Ritsu was sick and asking Mio to stay with her until she falls asleep.
And a reversal of that is when Ritsu was meeting with the president of Mio's fan club and the others followed her (at Mio's insistence) because they thought she was meeting up with a boyfriend. The best part comes when Mio begins crying and says "Don't do this, Men are animals".
In the movie when Mio is asking her mom if she can go to London, she says "Ritsu is coming too so it'll be okay."