Brandy Jumper, a 33-year-old teacher in Frisco, Texas, took the 17-year-old to the tattoo artist, who operated out of a suburban house.
The tattoo, between the lad's shoulder blades, featured three stars above his name, "Hunt".
But it is illegal in Texas to tattoo a minor. School and health department officials are investigating after Davonne Hunt, the boy's mother, complained to police. Ms Jumper has now been placed on administrative leave by Frisco High School.
Ms Hunt told a local newspaper: "It's illegal for him to get a tattoo, and he got it, so I'm really upset about that. They both knew he wasn't allowed to get one."
She said her son would be tested for infections that could be picked up from the tattoo parlour. And, like his teacher, he is grounded.