I hope you get to see that group of individuals that (have/has) just committed the crime.

Possibly an explanation of which correct word to use? I am really stumped.

Is the word principle correctly used?
the principle of the school takes a decision to correct the situation.

It is "has" because "group" is singular. If you took out "of individuals," it would sound correct using "has."

No! The principal of your school is your PAL, that's how I teach it to my students.