I showed up to a small, discussion based philosophy class tripping pretty hard on acid. It was pretty noticeable, I'm sure, from my breathing and from my expressions. The kid sitting next to me, who I had never talked to before, turned to me and said, "Dude, what are you on?!" I quickly responded "Nothing." and then wished I would have told him, so I could gauge his reaction.
The next semester I showed up the same class, again tripping acid. I had a different prof who turned me off of the subject entirely so I was content to stare at the patterns in the ceiling for the entire class. I mean the entire class, to the point that everyone took notice. I was pretty content looking into those patterns and didn't really mind.