I was wrongfully issued a check by the university I attend for overpaying tuition. They added a zero to the amount I paid somewhere down the line and I was mailed a six figure check for the difference. Being of high character, I alerted them to the mistake and it was corrected.

Assuming I tried to deposit the check and it cleared. Legally would I have to return the money when they caught their mistake?

Not only would you had to repay it, accepting it and keeping it ( in some areas) would or could be criminal, so if you did not pay it back or had spent it, you may have been facing criminal charges.

I was wrongfully issued a check by the university I attend for overpaying tuition. They added a zero to the amount I paid somewhere down the line and I was mailed a six figure check for the difference. Being of high character, I alerted them to the mistake and it was corrected.

Assuming I tried to deposit the check and it cleared. Legally would I have to return the money when they caught their mistake?

Yes, if they had caught their mistake, after you deposited the money, you would have had to return it, otherwise it would have been theft.