Kyle, you are right, on the day of my second visit it was graduation time, lots of students partying, but no trouble or aggro lots of the students were quite drunk, but well behaved, in its own way quite boring!
I visited Heidelberg on a school trip in 1973 IIRC. I dimly remember eating Wiener Schnitzel, and being guided around the castle by a very disreputable looking chap in a tatty black leather jacket. Oh and it rained all the time. Happy days.