USS Valiant (NCC-74210)
The USS Valiant began service as a training vessel for the elite cadet corps Red Squadron at Starfleet Academy. It was caught within enemy territory when the Dominion War broke out. All the officers were killed in battle and the dying Captain Ramirez appointed Cadet Tim Watters to command the ship.
Due to the ship's orders to maintain radio silence, Starfleet was unaware of the deaths of the ship's officers and continued issuing orders during the war, including ordering the ship to collect information on a Dominion battleship in their vicinity. Watters -- despite his inexperience and incomplete training -- decided to carry out these orders instead of returning to base or breaking radio silence to notify Starfleet of their condition.
After observing the target and obtaining sensor readings, the cadet crew of the Valiant subsequently exceeded orders by attempting to destroy the battleship instead of getting their observations back to Starfleet. Having a crew of nothing but cadets and being vastly out-classed, the Valiant was destroyed.