I always like to try and view both sides of an argument. Let's start with why body armor shouldn't be allowed on school grounds.
School should be a safe place where student's shouldn't be reminded that their lives can be in danger. Having one student with body armor, even if its hidden, has the possibility of causing confusion and questions among other students, parents, and school staff. Any one of these people might ask, why do they need body armor? Do they know something we don't?
Because of this some people will be confused as to why someone would need the armor plates or inserts to protect themselves. It's understandable.
On the other hand we know that armor plates can save lives. Is it better to have a paramedic on scene after someone gets shot or to buy another bulletproof insert. Would you rather bleed or feel like you got punched? At least for me I'll take the punch. I'd much rather have the safety that hidden soft body armor can bring.
Personally, I like the soft body armors. Steel you might ask? Yes, its cheap and all, but its simply too heavy. I have a bad back and I need something light. Having an insert that weighs less than an Apple MacBook Air that will stop bullets is easily the best choice for me. Especially when the same protection from steel would have been 2 or 3 times as heavy.