After inserting the three keys to activate the elevator and open the path to the final area, I returned to the base and checked the command room screen.
Despite the fact that the final mission status was listed as "In Progress," a "Complete" button appeared below it. Pressing the "Complete" button here ends the mission.
Lauri then disappeared from the path leading to the school, which prevented further access to the school and thus prevented me from seeing the ending.
I confirmed that as long as you don't press the "Complete" button, you can return to the school and continue the mission.
However, I believe the following issues are likely due to the developers not having enough time to address them: the inability to return to the school location after the ending; the incorrect description of the path leading to the school as "Romashka Cooperative"; the malfunctioning map location display while in the school area; and the school area not appearing on the map.
I hope these issues will be fixed soon.