Although I agree with the addition of achievements, you're essentially asking Jashinn to rewrite the whole game at this point.
The ripping and tears of clothes and other cosmetic effects I think would also suit well, but I think changing a lot of elements would be way to overwhelming and just being able to ignore the boss in the gym without a fight is underwhelming.
Also, as for how the teacher got the gun, most likely it was a concealed carry and because he was an adult he mostly likely also had a gun license (according to official Japanese gun laws.)