OK, so presuming you have your cron set up properly, then perhaps it could be one or more of the following:
When students are added to your site, are they added with their email enabled? If they are added with mail disabled, then of course they won't receive mail. You can check this by looking at the preferences for mail in each student's profile.
Are the students subscribed to the forum? If not they will not receive mail - the teacher can force subscribe students in the initial forum settings.
Are the students' mail addresses correct? So often they input incorrect mail addresses.
Hopefully you will resolve your problem soon.
Eric