1. A boy should be in school.
2. A boy should be at school.
(Are both the same? Which preposition do we hae to use?)
3. Get in the truck!
4. Get on the truck!
5. Go in the truck!
6. Go on the tuck!
(Can we use 'on' instead of 'in'? What about 'go' instead of 'get'?)
1 and 2 -- Either preposition will work fine.
3 - 6
You need to use "in" if you mean someone should get inside the truck. ("On" means on top of!!)
"Get" implies the person will be a passenger. "Go" implies the person will be the driver.