The University Grays reportedly got the farthest on Cemetery Hill and literally none of them weren't wounded or dead. Likewise, while I was a student at Mississippi College, the monument there to the MC Rifles reported that only 8 of the original 104 students who had enlisted returned to Mississippi.
I know that a ton of people died in the Civil War, obviously, but those levels of losses seem kind of insane. Was this typical? Or is this a reflection of the fact that maybe sending a bunch of rich college kids to shoot muskets was a bad idea or what?
Your two examples show two different types of casualty rates, short-term within one engagement (The UG's at Cemetery Hill) and long-term over the course of the entire conflict (MC Rifles). While the UG's have an atypically high short-term casualty rate, the MC Rifles' casualty rate over the course of the conflict is not surprising for the following reason:
Both of these college companies, like pretty much all university-based units, joined the war at its earliest stages. This means that these companies fought through all four years of the war, a circumstance which led to huge attrition rates. For example, then Washington College's (now Washington & Lee) Liberty Hall volunteers had only 2 of its original members paroled at Appomattox out of an original 73 who had mustered in Lexington. It is not surprising that the Mississippi College Rifles had similar casualty rates.
The case of the University Gray's at Cemetery Hill is atypically high, but can be better explained within the special context of Gettysburg as opposed to the idea of "Rich college kids shooting muskets" making poor soldiers. In his history of the battle, historian Allen Guelzo recounts that sustainable casualties for any one unit hover around 30%, any higher than that and the command and control system will generally begin to break down.
The UG's were Company A of the 11th Mississippi, the unit that comprised the extreme left of Pickett's Charge. This left it exposed to a savage enfilade fire from Union artillery and infantry. According to the regimental history only 40 men were left unwounded within the entire regiment. So while the UG's suffered grievously, it should not be marked up to it being comprised of poor soldiers, but rather of being placed in a particularly savage part of the battlefield - these men had after all by the point been engaged in over two years of war and fought in most of the eastern theatre's main engagements.
For comparison, the Battle of New Market in the Spring of 1864 saw the VMI Corps of Cadets (students who did not have near the combat experience of the UG's at Gettysburg) play a critical role in the battle's final phase. The cadets, however, suffered a causality rate of 23%, much closer to Guelzo's sustainable loss rate of 30% than the UG's rate at Gettysburg.