@Dan, yes, "male student and female student" is correct grammar. It could go either way though, the other option being "a male student anad a female student". Both are grammatically correct, and Duolingo accepts both. @[no name - penguin head profile picture] "male and female students" is not correct, as neither "студент" nor "студентка" are plural. As @Mirka Danylenko said, add the letter "и" on the end to make it plural (or in the case of the female, remove the "а" and add an "и").