You shouldn't worry about things like that. There are people who will call you a racist if the criminals are poc, but also if you feature more whites in order to make them the criminals. There are a group of people for whom "being outraged" is now a life pursuit. And if someone wants to be outraged, there is literally nothing you can do to keep that from happening. They'll find an excuse. So don't write for them, ever. Write the story you want. Do it well, and you will gain fans. Real ones.
As for your premise, I think it sounds great. A teacher working in the inner city and solving crime would be a unique twist on the usual story of the put-upon teacher in the inner city. Should the criminals be mostly poc? In that setting, how could they be anything else?
Uhm, what? I live right here in the inner city, and I've heard tell tale of white criminals. It's not even about racism, it's about what makes sense. A story about the inner city where somehow the gangs are full of white kids in polo shirts?
I feel like you're missing the point. First, gangs are kind of specific. My inner city is not so much with the gangs. However, there are gangs of all stripes, and just saying 'well there's a drug ring, so it's a gang, so it's PoC, doesn't make sense. You know who sells more drugs? White folks. More than 30% more likely to sell than black folks, among those aged 12-25. Soooo...yeah, drug dealers? White kids in polo shirts.
It's so out of touch with reality that people would simply be confused. Stick with what you've got. And, as I said, do not worry about the outrage mob.They're loud, but small, meaningless, and certainly disingenuous.
Good luck with the book!