Ummm...yes, and no.
Assuming that he also wants to get the information *per* professor and *per* day, I'd do this:
Though it strikes me as more than strange that there would be only *ONE* datetime_column. I'd expect one for checkin and one for checkout, even given the scenario described.
SELECT professor, DATE(datetime_column) AS theDay TIMEDIFF(MAX(datetime_column), MIN(datetime_column)) AS timeSpent
GROUP BY professor, DATE(datetime_column)
ORDER BY BY professor, DATE(datetime_column)
Somehow, I figure this has to be homework. And maybe not completely understood homework.