* Country: Educational systems and compulsory schooling laws varied dramatically across the world in 1930. Many countries had significantly lower rates of school attendance than today, especially for secondary education. Some nations had virtually no universal schooling.
* Definition of "left school": Did this mean graduating, dropping out, or something else? Many students in that era may have left school due to economic necessity, family needs, or lack of available opportunities.
* Age group: Were we talking about primary school, secondary school, or higher education?
To get an estimate, you would need to research individual countries' education statistics from around 1930. Even then, the data may be incomplete or unreliable due to limitations in data collection methods at that time. Many historical records were not as comprehensive or standardized as they are today.