You can do this yourself. Check with the clerk of the court in the county where you were convicted and ask for a Penal Code 1203.4 packet; you may have to pay ten dollars or so. You can download the form from www.courtinfo.ca.gov and type directly into it with Adobe Acrobat Reader, but the county form packet will provide additional information on the steps you need to follow, so I would get one even if you use the downloaded form.
The court clerk will also provide the numbers of your old case.
Read the school application carefully; it might tell you whether this conviction must be disclosed. If it isn't specifically required, I would say you don't have to disclose it.
Keep in mind that this really doesn't expunge anything. The court records will be amended to show your convictions were vacated and the case was dismissed. You will still have to disclose the convictions if you are applying for a professional license, and the records may be available to future employers in certain industries. If you are charged with petty theft in the future, your dismissed theft convictions may also be used to elevate the new case to a felony.