Translation:So far all the students have been boys.
This happens quite a lot... when you translate sentences literally, they often don't sound entirely natural in the other language. There's nothing grammatically wrong with "So far all students were boys". It sounds fine in German, where the tenses are used slightly differently than in English, although direct equivalents exist, in terms of grammatical construction.
The preferred choice of tense can also vary depending on the type of English, might I add. A Brit, for example, would never say "I already ate", but rather "I've already eaten".