A drill instructor, training instructor, company commander, or top sergeant was a non-commissioned officer who trained new recruits during basic training. The Vori title for this position was driller.
Hoshi Sato was released from Starfleet Training Command for bad conduct after breaking the arm of her company commander when he stopped the floating poker game she ran on the weekends. (ENT: "Observer Effect")
After Shahna informed James T. Kirk that she was his drill thrall in 2268, and that she would train him well, Kirk replied, "Yes, I'm sure you will. I must say, I've never seen a top sergeant who looked like you." (TOS: "The Gamesters of Triskelion")
After reviewing Crewman Darwin's Starfleet record in 2372, she found that his training instructor "recommended him as an officers candidate," but that he turned it down to to join USS Voyager. (VOY: "Meld")
When Quark and Rom attempted to recruit assistance for rescuing Ishka from the Dominion in 2374, they asked Nog, who initially refused, based on the fact that he was "a Starfleet officer, and as such, "I can't go running off with you on some half-baked rescue mission. I have duties to perform." Though Rom pleaded with his son to join, because he knew how to fire a phaser, Quark eventually turned to reverse psychology, telling him that it was too bad he wasn't joining them, because they were going to make his their "strategic operations officer [...] exactly like Commander Worf." Quark explained that Nog would "devise the tactics, come up with a plan of action," before Rom added, "you can be the team's drill instructor. Put us through our paces. Prepare us for combat, with Quark adding, "Turn us into soldiers." Finally enticed, Nog replied that "That's a big job," before finally agreeing to join their cause. (DS9: "The Magnificent Ferengi")