UUUUUUUUUU Why "you all drink tea" I know it is plural but never heard such translation. In ENglish plural sounds same as single in this case so "you will drink tea" should be accepted. The most frustrating is the fact that in some cases both forms are accepted by the system, in some cases only one of them, not always the same.
Because the sentence is "you (pl) will drink tea", not "you will all drink tea". The course allows you all as the plural you, since a lot of people seem to complain when its not accepted in other courses.
You works perfectly fine for most (if not all) instances of the plural you. If you don't normally use you all then don't use it here, and if you find a sentence penalises you for not using it, report the sentence.