Military School is a feature in The Sims, console versions and The Sims Bustin' Out. Children whose grades stay at F for too long are sent to Military School. The Military School is never seen in game, so children who are sent there will never be back. In effect, being sent to Military School is the child equivalent of an adult leaving after losing a fight, so there is no way to get them back, other than to exit without saving. Once the lot is saved after the child has left, the child is completely removed from the game, and cannot be recovered, even with cheats.
The day before a child is sent to Military School, they will get a warning saying that their poor grades are putting them "into the express line for military school", and that they will be sent there if their grades are not improved quickly. The Sim child must go to school the next day in a good mood, or raise their grades through study, or else they will be sent to military school.
When a child is sent to Military School, a dialog will appear at 11 am, saying that there are no more second chances and that military school is the child's next stop. After a few seconds, the camera will automatically zoom straight to the child, who will change into a grey military uniform and give a salute, then walk off the lot. This can be delayed by stacking up interactions, as children will only leave for Military School if they have nothing else in their action queue.
If Cassandra Goth, Daniel or Jennifer Pleasant is sent to military school, a fresh reinstall of the game will bring them back, but created children will never come back no matter what.