Does your 12 year old really have $100 you don’t know about? And ways to go spend it without your knowing?
Mine does not.
Where would he get $100?
Oh, school meals in the US are typically paid for with the school fees and students don’t handle the money, right?
I think this is going OT so I won’t continue to discuss here.
As we have heard, so have we seen in the city of the LORD of hosts, in the city of our God: God will establish it for ever. Selah.
Psalm 48:8, KJV
Yes, good thing. 
So basically with a professional level of computer expertise and a willingness to spend a lot of time configuring and managing the system, you can effectively force a 12-year-old to go to his friends’ houses when he wants to peruse porn.
It sort of reminds me of my nephews - my brother-in-law was a health freak. Besides the garlic-a-day stuff, he had no sugar in th house except a small amount of cane sugar for coffee; no sugar treats or candy for the kids. I remember visiting when they were ten, they would go down to the store with their friends and each buy a two-litre of the cheapest sugar-water soda and drink it in the garage in one sitting.
In the end they’ve turned out smart, healthy, and quite thin anyway.
Or the co-worker who proudly told us he didn’t let his kids watch immoral rude and suggestive TV shows - whereas many of their friends, I’m sure, had access to TV and videotapes with minimal parental guidance.
All you can do is point your kids in the right direction, and I’m guessing your overdesigned computer systems are just a symptom of good parental guidance. For the less technical, the “computer in the common areas” is probably the best solution.
He attends a school without a hot lunch program. He brings his lunch every day.
Well…I don’t like model trains.