Muggle-Born Eastern Europeans

Uh...what happens to them? Do they just never get an education? Or do they get shipped to another school?

No one from the school or ministry would come and explain that they were magical, so it would be highly likely that they'd live their life as a muggle. They'd probably receive a muggle education. Hope that helps

~ Vannah

As the area feeding into Durmstrang is rather large (Eastern Europe + Scandinavian regions + the eastern countries of Central Europe + Southeast Europe minus Greece + Caucasus regions + Russian Far East), it is more than likely that there are other wizarding schools that they can attend, though they may not be as renowned as Durmstrang. If their parents are well off and know where to find them, they might even have tutors if they're not near one of the other schools. Or they just don't receive a magical education.