As a teacher I am considered an exempt employee with a contract and on a salary. The contract does not specify the times to report to work, however the staff has been told that our hours are from 7:30a-3:30p. A number of staff members have been docked for arriving 10-15 min late, but we are never paid overtime for the hours past 3:30 spent. In addition, as a counselor, I have worked for the past 4 years up to 20 days past my end of contract. This was not a problem until this year when a decision was made that no longer would time worked past the contract would be "comped" out.
I questioned the days I still have from this past summer and was told that was part of my job, and as an exempt employee I was to work to get the job done.

If I'm an exempt employee paid to get the job done, if it takes 3 hrs or 3 months the same amount of money, how can I be docked for being 10 min late but not paid any overtime?

You can't. It is illegal to dock an exempt employee in partial day increments unless they are on FMLA.

However, while they cannot dock you for being ten minutes late they can require you to use ten minutes of vacation or sick time to cover it.