General help

Yep - change the subscription mode to 'Forced'.

https://docs.moodle.org/35/en/Forum_settings#Forum_subscriptions

Do note that it is possible for students to select digest messages or even disable messages completely so do not see this as a form of reliable "instant messaging".

You can force subscriptions in the forum settings which will notify all course users. Whether the user gets emails by default is in Plugins / Message outputs / Default message outputs. If you don;t want students to turn this off, you'd have to change their capabilities for notification preferences.