I received my first offense DUI while driving under a controlled substance, is it possible to reduce these charges or can they be expunged?

DUI's are harder to expunge than other crimes because of all of the press about drunk driving incidents.  However, it is possible to pled guilty to a lesser charge which would have fewer effects on your future. (like poss of marijuana)  Whether or not that this can happen will depend in part on the prosecutor and the judge handling your case. The strengths and weaknesses in a case will often influence the decision by the prosecutor.

However, before you do anything, you need to review the offense report, look at the blood test results, and make an informed decision.  You really want to do this with an attorney so that your rights are protected.  You mention in the end about "should they have read me my rights"-- if they asked you questions during field sobriety tests, then they would not have had to read your rights.  If they interviewed you after they arrested you, then yes you should have received Miranda Warnings.  An attorney can help you decipher what rights, if any, were violated and when.  With this evaluation, you can make a better decision about how you should proceed forward.  If you cannot afford an attorney, approach the court about having one appointed for you.  If your goal is to be a teacher, it is extremely important that you put the time into these details so that you don't hinder your future career objectives.