I laughed a lot on finding this in a bookshop last time I was in Japan, so I bought the first volume. Tomoko read it first and said it was rubbish. Unfortunately I sort of agree, although I'll still be checking out the anime to see if that works better.
It's a parody of Attack on Titan. In the horrific and extremely violent original, mankind is under siege by man-eating Titans anywhere up to 60 metres tall. In the parody, though, everyone's thirteen years old and attending the same junior high school. The familiar characters are all present, but with all the tragedy swapped out for silly incidents that make them look like idiots. (This is the point, obviously.) Eren still hates Titans with a fanatical passion, but not because one ate his mother. Instead he had one throw an eraser at him, while another stole his packed lunch. It had a cheeseburger in it! All Titans must pay!
Alas, I didn't really find this funny. Maybe I will in the anime? It came across to me as mistaking mockery for comedy. I don't have any religious attachment to the original, but by the same token I don't really get anything from turning Eren into a delusional prick. He's a twat. Okay, and? To be honest, characterisation wasn't the reason the original became an international blockbuster either. My favourite human-related bits were the ones where these school-age versions of the characters managed to come alive in their own right, e.g. Mikasa's bizarre fixation on Eren (to the point of losing brainpower and the will to live when he's not with her) or Annie being in tears in her game of near-homicidal dodgeball.
The Titans are funny, though. Any scene with a Titan in it is a laugh. They're monstrous shambling nightmares... at school. What's not to like? (I don't think they eat people in this continuity, though.)
Maybe I'd like it better if I rewatched the original's anime, or read its manga? Maybe. I think it's often replaying scenes from the original, almost beat-by-beat, but with everyone's backstory and motivations subverted. It also seems that quite a few people enjoyed the anime. I won't be buying any more of this manga, though.