News Forums are "special" forums. Students can not reply in them. http://docs.moodle.org/en/News_forum
Unless of course you want them to be able to reply in which case you can manage this via the permissions system, either for the whole site or for the news forum on the front page or for specific news forums within courses
Do you want students to be able to reply? If so in what circumstances (as per suggestions above) and are you comfortable with making changes to the roles/permissions mechanism?
Fair enough what Dave says -but if you want them to reply, why not just use one of the other forums where students can reply without the need to alter permissions?
Agreed, unless there is some specific reason you want to use the News Forum. I don't like messing with roles but changing the permissions on just that activity, like Dave mentioned, would be a good solution.