Hi Sheila,
This setup looks correct, but there are a few other things to check.
Since this is the front page, things are a little different. Users do not have the same roles that they do in a course. Users are normally assigned a role as part of their enrolment into a course, or course category - there is no enrolment at the site level, so only system roles apply - e.g. Manager, Authenticated user, etc. Therefore your students don't actually have the role of Student here and it cannot be applied.
The other thing to bare in mind is that forums on the site page are normally publicly visible. If I visit the site forum on my test site, setup as you've indicated with groups and a grouping, I can see a list of all of the groups and I can view posts in all of them.
That said, I can't view all of the groups after I log in.
There are two things which govern which groups you can see:
- group membership - so it's worth checking that they are only a member of the groups you are expecting them to be in; and
- capabilities - specifically the "moodle/site:accessallgroups" capability. I'd recommend checking the user permissions for this capability within the forum, and checking for any overridden capabilities, etc.
Andrew