Translation:High school students
Japanese doesn't usually distinguish between plural and singular nouns; usually you can get these from context. However, 'tachi' puts a little more clarity therein, it kinda means '-and company'. Like you get 「ねこたち」(neko tachi) which is more or less like 'a cat and company'. This たち doesnt necessarily require it to be the same object, either. For example you could say 「犬たち」and mean 'a dog and whatever is with it'. Context as always
中等学校 does exist, meaning "secondary school", it is a school where middle and high are integrated so it is 6 years of schooling rather than 3
The difference in abbreviations most likely largely has to do with the concept of 高等学校 being established later than the others. The first two schools are mandatory education, your primary learning and your secondary learning; whereas high school is optional. A school only a few select people attend would be considered "high-class".
I read your comment and finally understood the meaning of that question. '高校' is an abbreviation for '高等学校'. We use almost '高校' in daily conversation. '高等学校' is used to official document. The resume etc. Therefore '小学校', '中学校' and '高等学校' are including the word '学校'. Or the letter '校' already means school.
But '大学' is not include '校'. '大学' is '大学' alone.
how to read 高等学校（こうとうがっこう） 大学（だいがく）
They really should do something to eliminate the irrelevant definitions until they're needed in future lessons. I'm just going by contextual Clues but a lot of people are getting mixed up by them. I understand that this app is doing the best it can and it's only has Japanese on the app and not online but a lot of people need more definition with these types of languages
I know it's been forever since you've asked but in case anyone else is wondering too: Kanji can be read in multiple ways, with On readings and Kun readings. By themselves they take Kun readings. In a string of kanji to make up a new word they use their On readings, that's the "sei", but since you click on the individual kanji Duo reads it as the Kun reading of what this kanji sounds like when it stands alone, not the sound it makes in the context of the full word.