[Rodamala]

This is a serious and perhaps ignorant question for me to pose, but what prevents ordinary citizens from detaining a suspect, placing charges against the suspect, convening a Grand Jury, arraigning the suspect, gathering evidence, calling witnesses, holding a trial, and imposing a sentence? (Assume all participants slept at a Holiday Inn the pervious night)

[The Pack Knight]

This is a serious and perhaps ignorant question for me to pose, but what prevents ordinary citizens from detaining a suspect, placing charges against the suspect, convening a Grand Jury, arraigning the suspect, gathering evidence, calling witnesses, holding a trial, and imposing a sentence? (Assume all participants slept at a Holiday Inn the pervious night)

The law prevents it. Those citizens would be guilty of a number of crimes, starting with kidnapping. Any trial they held and any sentence they imposed would have no legal force.

[LoneRangerMassachusetts]

but what prevents ordinary citizens

The amount of arms at their disposal. They will need more than the state and the will to use them.

[nmh]

I'd bet you that students aren't allowed to carry any gun. They most likely have a “gun free zone” that criminals don't respect. These NUTS have guns. Universities typically don't.