A classic machinist apprentice task is to take a rough steel cube and make the sides flat and smooth using a hand file. That teaches you how steel behaves and can be worked. Life was slower a century ago.
High school students can learn CNC with smaller machine tools. There are little desktop CNC machines in the US$1000 range. You can cut aluminum, brass, and plastics, but not steel.
They talk the same G-code as the big machines. You design jobs for them the same way you do for the big machines. At that scale you can usually avoid coolant, oily rags, oily chip disposal, and the general mess of a real machine shop.
Taking that lesson to its extreme you realize there's a rounding effect where seemingly no matter how carefully you hold the file, the edges of the face are cut down more than the center. To combat this you might discover you can use the end of the file (like the last tooth) as a scraper. You can use a gauge block and some blue ink to discover the high spots and repeatedly work them until it's flat. Congratulations! You've just discovered how to make a lathe bed.
High school students can learn CNC with smaller machine tools. There are little desktop CNC machines in the US$1000 range. You can cut aluminum, brass, and plastics, but not steel. They talk the same G-code as the big machines. You design jobs for them the same way you do for the big machines. At that scale you can usually avoid coolant, oily rags, oily chip disposal, and the general mess of a real machine shop.