I imagine it should be possible to set up an automatic welcome message that is sent to new members with this info when they set up an account. They may be more likely to see and read that than the forum post.But a generic message when posting like "Duu to a large number of spam attacks in the past, we need to make sure you are not one. Please allow a few days until this gets approved." will do wonders. Again, the thread you made for new students is a step in the right direction.
Also @Jak Angelescu I don't think the issue is you taking a break, it's that there is nobody to step in when you do.