The beginning held a "promising" set up, as our high-school girl protagonist enters the dark, illegal underworld of assassins. Although she's become one of them, she refuses to join their organization, thus sparking a big competition for pro assassins to go after her head.
Now bear with me... You have no living relatives, no friends, no money, no roof over your head, no valuables to sell, and barely any food/water left. All you have is a special weapon for performing assassinations, the skills to use said weapon, and a bounty over your head that has assassins coming to kill you in the future. If you were in this situation, would YOU go back to high school the next day, not worrying about your current situation? Well, that's what the protagonist does.
Maybe she did it to "blend in" with the other students, as after all, it is a public place.
However, the series has currently become a cluster-fuck of numerous shounen-style battles, one after another, that are all taking place in her high school during the day.
Even when she's given the opportunity to leave the damn high school and prevent the assassins from going after her, she refuses because she's worried that one of her friends is still in the school. [Sigh...]
Take something like Darwin's Game or Underdog, both about a protagonist that must learn "to kill or be killed" as part of a deadly game he gets stuck in, making anywhere he goes in Japan unsafe to tread. That gives the opportunity for the protagonist to meet +interact with all sorts of new friends and foes in different kinds of environments.
Arachnid though can't seem to stop clinging to its incredibly standard high school setting, having each and every battle take place in either an empty classroom, an empty nurses office, an empty hallway, an empty pool area, etc. And remember, this all takes place during the damn school day...
I'll admit, I do like some of the things the series does, where it has a female lead, insect/arachnid-themed powers, and several of the character designs that seem pretty interesting. Still, I would certainly not call it a seinen, given how poorly the characters develop, the ridiculous number of battles that seem to act like that of any other action-based shounen series, and the incredibly preposterous ways the characters manage to overcome the odds against them in a fight...