From what you have stated the DA decided not to file charges against you for the underage drinking and driving. If the DA try’s to go back and add the charges they will be preempted due to your guilty plea on the traffic violation. The issue you have is the restricted license. It is very difficult once the DMV hearing has passed. The DMV does not like to give restricted licenses to people that are under 21 and were caught with any alcohol in their system.
Here is the best way to improve your chances for DMV to grant an Application for a Critical Need Restriction. You do not have to be enrolled in DUI classes since you were not charged or convicted of the relevant statutes. Enrolling in the classes does not enhance your chances. Just follow these instructions:
As far as your license, since the 10 days has expired, please print-out and complete the DMV Application for Critical Need Restriction found at http://www.dmv.ca.gov/forms/ds/ds694.pdf . Include as many documents as possible from your school, physician, &/or employer to support your need for a restricted license.
DMV's guidelines are found at http://www.dmv.ca.gov/about/lad/pdfs/cnr/terms.pdf .
In order for the Application to have any chance of being granted, you have to get as much documentation as you can - showing no transportation is available for the time(s) you go to/from school &/or work.
Attached public transportation schedules, MapQuest maps from home to school &/or work, etc. in order to show it's impossible (not just inconvenient). The more documents you attach, the better.
To give yourself the best shot, if possible:
1) Submit local bus schedules and highlight possible timelines of travel.
2) Use maps and MapQuest and/or aerial views of the route to and from work to show that bicycles and walking will not do.
3) Submit affidavits/declarations (statements "made under penalty of perjury") by yourself and family members (and/or roommates) attesting to what you contribute to the family (or household) and why it is not possible for them to drive you. State no other licensed person resides in the household is available for carpooling. If you are working, state your income is essential to the household.
4) Submit letter from employer(s) indicating how valuable you are and the need for you to drive, and the lack of flexibility in your schedule.
Mr. Driessen and Mueller offer great answers as usual. The DA cannot refile charges against you again and you may apply for the critical need license, however they are very tough to get approved. Best of luck.