It's trash, but also surprisingly good. I enjoyed it and I'd even give it a low-grade recommendation, which I certainly wouldn't say for everything in its silly Japanese splatterpunk genre (Machine Girl, Helldriver, RoboGeisha, Vampire Girl vs. Frankenstein Girl, Tokyo Gore Police, etc.) Both positive and negative, the key things you need to know about this film are:
(a) it's quite amusing. The joke is that this is a crazed zombie robot action film whose main character (Giko) carries a chainsaw around school and almost all her classmates have been transformed into monsters... but it's also adhering to the rules of schoolgirl drama. Giko is trying to do a make-up test and tries to get zombies to let her past nicely because she doesn't want to chainsaw anyone in half. The crotch missile cheerleader gets a tragic backstory that she relates in a flashback. The ninja zombies will be concerned about needing three members to count as an official school club. There's a teacher who has to approve all incoming students at the start of the day and can confiscate anything that's against the rules. "How many times do I have to tell you not to take your chainsaw into school?"
(b) those aren't zombies, by the way. They can talk and so on. They're wacky SFX opportunities, e.g. the severed head that wanders around on robot crab legs to deliver letters.
(c) the acting's solid. No one's wooden or incompetent. In particular, Rio Uchida's putting a ton of energy into Giko and I'm not surprised at all to see that she has a respectable non-trash acting career in Japanese film and TV. I don't think she gives a single lazy line delivery throughout the film.
(d) there are panty shots, but the film's almost nudity-free. The only exception (and it's very brief) is a topless girl who catches a chainsaw between her boobs. This ends exactly as you're imagining.
(e) the gore is all cheesy nonsense and/or CGI. The colour red is underused. You could almost show this film to children.
(f) The film's oddly nice! Giko is a delinquent, but a good-natured one who tries to persuade zombies to walk away from fights and then might give them medical aid afterwards. "Lie still, you're injured!" I also liked the film's last scene, in which she's polite and respectful towards her cool dad.
Above all, though, the film's a laugh. The schoolgirl zombie-maker (Nero) has a silly reason for her revenge rampage and builds a dead-yet-alive cat in a box. (Hurrah for Schrodinger!) Engineering Club Girl gives Giko's chainsaw extra functionality for comedy and makes some amusing observations about delinquents. Don't expect this film to change your life, but it's short, refreshing and significantly above average for its genre. (That said, though, Hiroki Yamaguchi made two Bloody Chainsaw Girl Returns sequels in 2019 that I probably won't seek out: "Giko Awakens" and "Revenge of Nero". I enjoyed what I watched this morning, but I don't think I need to see 2-3 hours' worth of follow-up...)