Since there aren't any SOQL queries in your trigger, the error is coming from the OpportunityCloneWithItemsController constructor (or some function called by it).  Is the constructor setting the value for Hidden_New_Class_Start_Date__c?  If not, do you have to cast the Opportunity?

I was really hoping it wouldn't be the class.  Here's the class code.  The class does reset the hidden class start date field back to null just after it inserts the new opportunity.  I think I have everything listed first and then set to query so I'm not sure what is going on...

Thanks so much,

Amanda

public class OpportunityCloneWithItemsController {
 
    //added an instance variable for the standard controller
    private ApexPages.StandardController controller {get; set;}
     // add the instance for the variables being passed by id on the url
    private Opportunity opp {get;set;}
    // set the id of the record that is created -- ONLY USED BY THE TEST CLASS
    public ID newRecordId {get;set;}
 
    // initialize the controller
    public OpportunityCloneWithItemsController(ApexPages.StandardController controller) {
 
        //initialize the stanrdard controller
        this.controller = controller;
        // load the current record
        opp = (Opportunity)controller.getRecord();
        
 
    }
 
    // method called from the VF's action attribute to clone the opp
    public PageReference cloneWithItems() {
 
         // setup the save point for rollback
         Savepoint sp = Database.setSavepoint();
         Opportunity newOpp;
 
         try {
 
              //copy the opportunity - ONLY INCLUDE THE FIELDS YOU WANT TO CLONE
             opp = [select Id, Name, Type, AccountID, Estimated_of_Students__c, StageName, Hidden_New_Class_Start_Date__c,Competitor__c, Competitors_in_Consideration__c from Opportunity where id = :opp.id];
             newOpp = opp.clone(false);
             if (opp.Hidden_New_Class_Start_Date__c != NULL) {
                 integer Month=opp.Hidden_New_Class_Start_Date__c.Month();
 
             if ( Month==11 || Month==12 || Month==1 || Month==2 || Month==3 || Month==4) {
                 newOpp.Academic_Term__c = 'Spring';
                 } else if ( Month==5 || Month==6 || Month==7)  {
                 newOpp.Academic_Term__c = 'Summer';
                 } else if ( Month==8 || Month==9 || Month==10)  {
                 newOpp.Academic_Term__c = 'Fall';
                 } else {
                 newOpp.Academic_Term__c = NULL;
             }}
             if (opp.Hidden_New_Class_Start_Date__c == NULL) {
                 newOpp.CloseDate = date.Today();
                 } else {
                 newOpp.CloseDate = opp.Hidden_New_Class_Start_Date__c;
             }    
             if (opp.StageName == 'Students Registering') {
                 newOpp.StageName = 'Prior Sapling User';
                 } else {
                 newOpp.StageName = 'Confirmed Teaching/Class Schedule';
            }
            if (opp.StageName == 'Students Registering') {
                         newOpp.Type = 'Renewal';
                         } else {
                         newOpp.Type = NULL;
            }
            newOpp.Hidden_New_Class_Start_Date__c = NULL;
             insert newOpp;
            opp.Hidden_New_Class_Start_Date__c = NULL;
             update Opp;
             
            
            
             // set the id of the new Opp created for testing
               newRecordId = newOpp.id;
 
             // copy over the line items - ONLY INCLUDE THE FIELDS YOU WANT TO CLONE
             List<OpportunityLineItem> items = new List<OpportunityLineItem>();
             for (OpportunityLineItem oli : [Select o.Id, o.unitprice, o.Quantity, o.PricebookEntryID, o.Description From OpportunityLineItem o where OpportunityID = :opp.id]) {
                  OpportunityLineItem newoli = oli.clone(false);
                  newoli.OpportunityID = newOpp.id;
                  items.add(newoli);
             }
             insert items;
             
              // copy over the Contact Roles - ONLY INCLUDE THE FIELDS YOU WANT TO CLONE
             List<OpportunityContactRole> Roles = new List<OpportunityContactRole>();
             for (OpportunityContactRole ocr : [Select c.Id, c.ContactID, c.IsPrimary, c.Role From OpportunityContactRole c where OpportunityID = :opp.id]) {
                  OpportunityContactRole newocr = ocr.clone(false);
                  newocr.OpportunityID = newOpp.id;
                  roles.add(newocr);
             }
             insert Roles;

         } catch (Exception e){
             // roll everything back in case of error
            Database.rollback(sp);
            ApexPages.addMessages(e);
            return null;
         }
 
        return new PageReference('/'+newOpp.id+'/e?retURL=%2F'+newOpp.id);
        
    }

}