Question on initial secret clearance

I just started a job and it requires secret clearance. This is my first time doing secret clearance. My question and concern is I got expelled from college about 20 yrs ago with several of my friends for playing around with grades. I transferred to another college to start over and graduated. Will this cause problem to get secret clearance?

Anything that happened 20 years ago has been mitigated by time as long as there are no more recent instances of similar behavior.

What does mitigated mean?

That the concern has been overcome by other factors that negated it.

Will they still investigate my college information even if I attended over 20 yrs ago? And do I need to explain what happened?