The Site Announcements forum is just another forum and has all the same settings that every forum has. The subscription mode can be accessed from the “Edit settings” page in Forum administration.

Set it to “Forced”, or “Auto”. Auto means people are initially subscribed but have the option to opt out. Remember on the front page forum that subscribers are not just students - it’s everyone on the site.

Which theme are you using? The settings links are in different places in Boost to some other themes.

And on the front page, do you have (or can you add) a Main Menu block? You can access the settings from there too. If it’s not there already - add it, use it to find the Forum settings, and then if you don’t want students seeing it, you can hide it again without impacting on the Forum.