The difference between ethnic and national schools is related to the languages teached at school. Taking the example of Australia, national schools are normal schools oriented basically to locals and others who can speak the native language of the country which is English. The courses and lessons are normally offered in English. However, ethnic schools are language and culture centres that are open to all students irrespective of their linguistic background. About 100 ethnic schools in South Australia teach a total of 49 languages after-hours.

I am from the United States and I have never heard either term used in any context.

Not exactly. I think it is affected by our different cultures. I just don't know how to translate it correctly into English. However, thank you all the same~∩_∩

Well, I know what a middle school is... I used to teach at one.... but the terms 'ethnic' and 'national' are not applied usually in the UK. So I can only assume that by 'national' you might mean a state school, and by 'ethnic' you may mean a private school that caters to a particular religion or culture? ???????