Hi Motth! I'm not an admin, but this is my understanding of the muggleborn student situation:
There are a few options for what your character could have been doing during that time. Most students either transferred to Beauxbatons (which would mean they could continue into the next year like usual upon their return to Hogwarts) or were on the run, perhaps abroad or in the "Ward", an MH-canon underground community where muggleborns hid out during the war.
If you choose the second option (on the run), your character's preparation for the next year would depend on what specifically they did with their time. If the student was on the run/living in the wild and focused mainly on surviving, they probably wouldn't be prepared for the fourth year and would need to do their third one upon their return. However, if they were in a more stable situation and had some sort of private tutor or alternative schooling for the year, they might be prepared to move on to the fourth year when they came back to school. I think you would need to explain what their year was like and defend why or why not they would be fit to be in the fourth or third year the next fall.
