Story: A great story, something different from the usual anime you see...shame there's zero plot involved and what is there is confusing and muddled up. In the end, we get more questions then answers. No idea if they mean to give this another season or not but as it stands right now, that's not a great way to end an anime.
Art: Decent and pretty at times. Some of the characters looked very nice while others looked just...weird (Ban and Hyo stand out as looking really weird). The school looks dull but that's to be expected as it's setting. Nature outside the school is pretty. The only parts I have issues with are the 3Dish models like in the last episode the elevator (jarring to have a 3D model in a flat anime) and the fire effects (looked horrible).
Sound: Decent, nothing to say here.
Characters: Oh geez, this is where it really falls flat. The characters are unlikeable and the one or two that you seem interested in get no ending to their story...we don't get to find out what happens to them! Why get invested at all?!
Enjoyment: If there was actual plot involved and we learned what happened to everyone in the end...and they answered some big questions...my score would be higher. The characters again are bad (some good) but I could live with them if I got some of my questions answered!
Overall: An interesting anime that started off well but fizzled out with lack of any understandable plot and more questions then answers. As others have said and pointed out...this is not a horror anime. Don't go into this looking for horror as I did. I stuck with it because of the interesting idea of a prison school for people who are addicted to the internet/gaming but...horror fans, be prepared to be let down. There is violence and blood but gore isn't really a thing here. So again, if you're into gore, probably not going to get your fix here.