system
Given the following post, respond as if you're commenting on Reddit:
human
[Star Wars] How would the events of the original trilogy be different if Luke did join the academy?
gpt
I'd be willing to bet that there are systems in place intended to recognize the Force-sensitive [which we've already seen in *Star Wars: Rebels*]. Luke would have quickly been identified as being quite powerful and moved into a special program for the gifted, which is actually a training course for special agents of Vader or Palpatine. Indoctrination would be strong, Luke would probably resist, but eventually would mostly succumb, keeping his dissatisfaction closely guarded. 

He'd be a loyal Imperial assassin, until he's tasked with killing the daughter of a Senator, someone who seems oddly familiar. In a snap decision, Luke would turn on the storm troopers, save Princess Leia, and flee back home with her and two droids. When he arrives home, Obi Wan would take him and his unbeknownst sister into hiding. 

In Mos Eisley, they'd secure transport on the Millennium Falcon to get the Death Star plans to Alderaan. Luke would make his move, but would be rejected (lookin' for love in Alderaan places?). They'd be tracked, despite Han's best efforts. The plans would be delivered, and the group would be guests of the Alderaan government, until they pick up a massive object on an intercept course. Sensing the danger, Luke would get everyone back on the ship and call for a planetary evacuation. 

The Falcon gets away, but some of the other survivors are caught and are interrogated, revealing the location of the rebel base on Yavin IV. Alderaan is destroyed. 

Luke would become captain of the rebel fleet, which makes a successful attempt on the Death Star. Luke would begin training under alive Obi Wan, both under the leadership of Yoda, on Degobah. 

Hoth would then be attacked and destroyed, with the Falcon leaving with the normal folks. Luke would still be training, but would sense the disturbance. He'd go into the psychedelic cave and kill himself in a Vader suit, but it would make a lot more sense, because this Luke would be ruthless and aggressive already, far more like Anakin once was. 

Luke would go to Bespin to save his friends, and be largely successful. He'd easily decapitate Boba Fett, and then attack Vader with fury and power. Vader would win the lightsaber battle, but barely. Sensing Luke's connection to the Dark Side, he would essentially lay his cards on the table. The fight would end, Vader would sit, and he would explain everything that had happened. Ultimately, Vader's plans about ruling the galaxy as father and son were not entirely about personal power, they were about the greatest tragedy of either of their lives, the death of Pademe. The Sith can learn to control life and death, to rule not for evil, but because order is all that can keep chaos and suffering at bay. It's convincing. Luke's condition of returning is that Leia is spared and Han, Chewie, R2, and C3PO to be released. He accepts the conditions. 

On the second Death Star, Vader introduces his son to Sidious, and the three engage in an epic lightsaber battle, tearing apart entire areas of the new station. Sidious largely holds his own until Vader gets in a very lucky shot by using a hidden weapon in his armor, injuring Sidious. In the end, Vader sacrifices himself to save his son, who kills Sidious by giving into pure rage (a reflection of Obi Wan reacting to the death of Qui Gon). The old emperor is dead, long live the Emperor.