It's not that bad, apart from a stupid bit at the end. It's watchable. However I see almost no connection between this and the source material it's supposedly adapting. Admittedly I haven't read the original novels, but I'm assuming that the anime's more faithful.
The title characters (Kanna Hashimoto's Chika and Shori Sato's Haruta) have the same names as the original Haruta and Chika, but that's about it. They're played by pop stars, of course, with Sato's boy band being the idol group Sexy Zone. (This is his first film, by the way, and his first acting role of any kind in four years. He's only twenty. Couldn't the producers have chosen an actor who occasionally does some acting?) In fairness they reach a basic competence threshold and you won't be throwing things at the screen or anything, but you wouldn't call them good and they have no chemistry at all. I didn't care whether or not they got together. Sato barely exists. Hashimoto hitting him doesn't suggest a childhood friendship, but instead makes Sato look pathetic and Chika rather unpleasant.
1. Hashimoto's Chika isn't in love with her teacher, isn't a tomboy and doesn't dream of being cute (perhaps because the actress is so cute already that any attempt to play that would have looked ridiculous). She switched from volleyball to the flute not for any character-based reason, but simply because she did in her knee. She's violent in a few early scenes that made me dislike her quite strongly, though.
2. Sato's Haruta isn't clever, homosexual or in love with his teacher. There's one scene where he says something intelligent and Chika says "how do you know?", but otherwise he's a personality-free zone. I only even knew it was him when he was standing alongside Chika. If you put him in a scene on his own, I wouldn't have noticed he was there. Admittedly this could be characterisation, since Chika remembers his childhood self as being a spineless wimp, but the Haruta I'd been expecting is a smug know-it-all who's rude to Chika all the time and deserves it when she occasionally hits him.
They're bland. They don't do anything Haru-Chika-y. They're just forgettable teenagers in a movie that wishes it was Sound! Euphonium.
As for the script, it's been pared down to something simpler and more mainstream. There's no detective work, really. It's not about the puzzles. It really is about Chika trying to play the flute in a high school band club. It's also doing that frustrating thing I sometimes notice in Japanese films (both live-action and animated) of dividing itself up into rigid episodes. The first thirty minutes are Act One. BANG. The clock's ticked, so it's time for a new episode! I do mean exactly thirty minutes, by the way. You can set your watch by it. The second thirty minutes will be Act Two, and so on. It's as if an obsessive-compulsive read Robert McKee and interpreted it in strange ways. That said, though, I suppose it's no different from watching episodes of a short TV series.
EPISODE ONE: Chika tries to recruit members for the band club. The headmaster's a dick, so he says she has to find nine people or he'll disband them. (Why nine? The magic number in anime always seems to be four. Well, I suppose a school can do what it likes... but it still feels odd since the club was completely safe in the anime even at the beginning with only five members.) This episode is fairly pointless and has all the scenes of Chika hitting people.
EPISODE TWO: is where the film actually adapts some of the source material. We meet Naoko Serizawa (Yuri Tsunematsu) and Kaiyuu Hiyama (Hiroya Shimizu). Serizawa works quite well. Her story's pretty simple. She's fairly unpleasant, but then we learn some information that makes us more sympathetic towards her. Hiyama, though, has lost something in this version. He's the one with the old folks' radio show, which is still a lovely idea, but here it's being undermined by the film being too obvious about it. It's a bad radio show! They're terrible presenters. They mutter, they allow long stretches of dead air and they sound as if someone smuggled a microphone into a nursing home. As a result, later there's no element of surprise.
Chika's stopped hitting people, thankfully, but even so I've no idea why Haruta's hanging around with her. That said, though, you'd hardly notice if he disappeared. The most significant character is actually a fat boy who doesn't like Hiyama.
EPISODE THREE: it's about band club. It's really all about band club. They've found nine members, so Cock Headmaster lets the club proceed as promised. The club members celebrate by physically carrying him through the school against his will, because as we all know it's safe to make an enemy of a vindictive, childish teacher who already dislikes you. Anyway, our heroes play music. They're not very good, so Kusakabe-sensei tells them off and they practise... but Chika still can't do it. (She's never played the flute before.) She's the weakest link. Will she succeed with her flute solo? What will happen in the national competition? Oh, and there's also some romantic resolution between Chika and Haruta, if you care.
This is pretty bland, but okay. There's one distinctive scene, in which Kusakabe-sensei takes Chika outside and the rest of the club slowly and shockingly degenerates into a mob. This takes several minutes and the camera never moves throughout. It just impassively goes on showing the whole classroom in long-ish shot, like a natural history documentary of apes in the wild. That was actually quite brave, but on the downside the finale has everyone in the club going up to the roof, barricading the stairways and serenading the school during class hours. Why? What's the point? Are they looking to be put in detention? They're encouraging Chika to play her solo and when she succeeds, suddenly everyone in the school is dancing and doing somersaults in wild celebration. This is because, um, we're watching a film and Chika's the title character. The entire school goes berserk. I wanted to kill someone when even Cock Headmaster started boogieing on down.
I've been rude about this film, but it's a reasonably competent high school flick that would hardly have annoyed me at all if I hadn't been familiar with Haruchika. Hashimoto is very appealing when she's not hitting people and I'm sure she has lots of fans. Sato barely exists, so it's hard to hate him. However this really is Haruchika in name only, with only the sketchiest connection to either its story or its characters. Haruta and Chika themselves are the biggest loss. You don't really notice them. You won't care that they changed Haruta's sexuality, because there's almost nothing of the real Haruta there in the first place. They don't exist as a couple, even when they officially do. They're just two nondescript people who are sometimes on-screen at the same time, with their characterisation gutted and played by a semi-actor and a non-actor.
Keisuke Koide's good in the underwritten role of Kusakabe-sensei, though.