jüriöö ülestõus (georgesnight uprising) a rebellion in estonia 23. april 1343 (in the night of St. George's Day) against germans. Estonian peasants killed many germans including children and women but they lost.
In slang is jüriöö when all (or most) students/pupils pack in the middle of the lesson their stuff, stand up, yell "jüriöö" and run out of the classroom (and usually go home). It is done when it is obvious, that the schoolday is long enough and has to be ended.
teacher writes: "... arbitrary left the lesson, absent without a reason, tomorrow detention we repeat the same lesson"