After the first engagements on Harvest, ONI Section II kept the Covenant invasion secret from a large portion of the general population, even as late as 2526. This included the cadets and staff of the Corbulo Academy of Military Science on Circinius IV. However, the Academy's superintendent, General Black, was informed. As preparation for a possible attack, Black discreetly increased the readiness of the UNSC Marine garrison posted in the academy and authorized limited communications blackouts under the guise of routine maintenance procedures.
On April 26, 2526, the Covenant began an assault on Circinius IV, targeting the Corbulo Academy of Military Science with three SDV-class heavy corvettes. Locally stationed Marines provided an initial defense of the Academy and the evacuation of the cadets via the school's space elevator was begun immediately. Orbiting UNSC vessels soon deployed Orbital Drop Shock Troopers directly onto the campus. However, one of the Covenant corvettes fired on the elevator, causing it to collapse on the school and halt the evacuation. Since all the cadets and staff had converged near the elevator for evacuation, the collapse of the elevator killed many members of the academy.
— John-117 notifying the cadets of the planet's near-total fatalities.
Corbulo's officer cadets were caught off-guard by this attack. General Black was killed within minutes of the attack; ColonelK.L. Mehaffey attempted to organize the surviving cadets inside the tunnels, though she too was quickly killed. Soon, only four members of Hastati Squad who had managed to take refuge in Corbulo's armory were the only human survivors on the entire planet. Cadets April Orenski, Thomas Lasky, Chyler Silva, and Michael Sullivan were rescued by SPARTANJohn-117, who then opened the cadets' lockers and allowed the cadets to arm themselves.
The Spartan led the survivors to the academy's exterior, where they were ambushed by Kig-Yar Snipers. After dispatching the snipers, they retreated from the combat zone in General Black's personal Warthog. However, the commandeered Warthog became inoperable after running over a Kig-Yar. At the same time, Silva was mortally wounded by a stray Needler round, forcing the consolidated group to retreat on foot. SPARTAN-117 engaged a Hunter, depleting all of his ammunition to buy the cadets time to escape. However, a second Hunter maintained pursuit, where Lasky volunteered himself as a decoy as part of a pincer maneuver allowing Master Chief to deliver a killing blow on the Hunter with squad's final frag grenade. Shortly after the battle, SPARTAN-117 and the cadets were evacuated by SPARTANsKelly-087 and Frederic-104. As the Pelican flew away, Kelly and Fred removed their helmets, showing to the cadets that they were indeed humans inside. Their youth and unnaturally pale appearance was a surprise to the cadets, but when asked they declared their age classified.