Millions of uneducated teenagers on the street without support from their parents or anyone else won't be solved by "hiring more police officers"
I'll try to explain this non sense in your language as you seem to be looking at the $ side of things without a moral perspective.
"More police officers" + millions of board teenagers on the street fending for themselves (gangs) will without a doubt place a massive surge on arrests and tax payer funded prisons.
That means instead of paying a single teacher to control and educate 30 students 6-7 hours a day, by your logic we are now instead accommodating, feeding, protecting and PAYING for an individual who will most likely grow up to have a better understanding of the inside of a jail cell than the real world.
Less educated, productive individuals out there slows down economic growth. The inevitable growth in crime rate will also effect the financial security of local businesses (mexico is a good example of this).