One thing that can happen when the native has personal planets placed in the 12th, is confusion/uncertainty with personal relationships and friendships.
For example, you say that you were not 'invited' to hang out with the other students. But did they know you wanted to?
Sometimes 12th house natives put out the 'message' that they don't want to hang out, or are not interested or open to that friendship.
Sometimes 12th house Mercury/Venus placements create a situation of hyper sensitivity. You might have been accepted and welcome to attend parties, but were too sensitive or insecure to ask, or seem interested?
This.
My observation, based on people I've read for, is that people with twelfth house placements are no more likely than anyone else to have hidden enemies for real, but there's a marked tendency for twelfth house placements to correlate with belief that people are plotting against you, secretly hate you, conspiring to exclude you, etc. One example that I recently saw: an astro friend of mine asked if I saw any reason in her chart why women are often jealous of her. She has a tight Venus/Neptune conjunction in her twelfth house. My response: I see that she believes it, but not that it's actually true.
I actually know about some of these specific situations where she's believed that people (women in particular) were jealous of her and/or plotting against her. From what I see, it's rarely if ever true. But she tends to feel it's true.
If you have Neptune in this twelfth house mix, or in aspect to it, then you may be in a situation much like my friend's: tending to believe that you have secret enemies. I suppose Pluto could also contribute to that, if it is, again, either part of this twelfth house stellium or in aspect to it.
And here's another ace to turnover: you said these twelfth house planets include your sun. Are any of your other twelfth house planets combust? (A planet is combust if it's within 8.5 degrees of the sun, and more markedly combust the closer it is to the sun.) People with combust planets have the repeated experience of being overlooked. Not deliberately, not maliciously, just overlooked, and often enough that it might feel like you're somehow invisible.
An astrology teacher of mine told a story on herself to illustrate this concept: she once had a class with several students who had the same sun sign, and it wrapped up during their birthday month. On the last day, she wished them all happy birthday personally... but overlooked the one with a combust planet. This, when she had all their charts and also knew about the tendency for people with combust planets to get overlooked.
That, again, would mean that people aren't deliberately excluding you, but your own experience tells you that you get excluded. These same people, though, probably would include you if you reminded them to. If you yourself invited your friends to a party, they would probably come. (Or flake or forget, because friends do that, but you likely would get some people partying with you.)
Where I would expect the twelfth house to indicate genuine hidden enemies is in horary astrology, if the question asked is relevant to hidden enemies. But that's quite different from natal astrology.