ragazze or ragazzi
In a school class i see some girls, but I do not know if some of them are a boy. What should I call them? Ragazze as just girls or Ragazzi as a mixt of boy(s) and girl(s)? and also I do not know if it one or two boy(s) in the class. therefor I cant say "ragazzo o ragazze". The same problem in French and Spain...
What would you call them in English? If you'd go up to them and say "hey, girls" do the same in Italian, the eventual boy will understand :P If you'd simply call them "people" you can use generic statements in Italian as well, from the semi-informal "salve gente" to a simple "ciao". Viaggiatore's suggestion is pretty sensible as well, and having them fix a mistake of yours is a way to break the ice :)
I find that hard to believe; the distinction of sex is different from that of grammatical gender, so much that languages with no grammatical gender or number like Japanese have two different terms for males and females of any age or relation. Whether it's ragazzo/ragazza, boy/girl, Jung/Madchen, shōnen/shōjo, you'll always have a three-way choice to make: male, female or impersonal.
well, you never say "hello boys" when you know that it's girl(s) in the group. But you can say "ciao ragazzi"
piccono is right -- in German you don't generally use "hello boys/men" / "hello girls/women" in a greeting, so you don't have that problem in this context. It is either "Guten Morgen" or in an informal context "moinmoin" or at most "Hallo allerseits" which is "hi everyone". On a different note, referring to a group of co-workers might be trickier, since a lot of occupational words in German are gendered male or female and that ends up with hilarious "Mitarbeiter/innen" or declarations that all the male nouns in the document also apply to females.
As I understand it, 'ragazzi' is used for a group of all boys OR a group of girls and boys. 'Ragazze' would be a group of all girls. Same goes for other terms, such as 'studente'. You can use 'studenti' to refer to a group of all male students OR a group of both male and female students. If it's a group of all female students, you'd then use 'studentesse'.