Hi,

This is super helpful, thank you! 

I modified the code slightly because there were a couple of extra fields I wanted on the Visualforce page. I also renamed the Session object, because the API name on the object itself was Sessions__c, but the API name for the master-detail relationship on Class_Enrollments__c was Session__c (without the "s" at the end of Session). I realized this was causing me issues, so they are now both Session__c (object API name) and Session__c (field name on Class_Enrollments__c object). 

I am running into a couple of issues:

1) I am returning this error whenever I try to preview the VF page: 
System.QueryException: List has no rows for assignment to SObject
Class.ClassEnrollmentsExtensions.<init>: line 9, column 1

2) I would like to pass a specific record type ID (0120f000001QfBc) to all records created via this button. How do I do that? 

Below is my amended code:

Class Enrollments VF:

<apex:page standardController="Class_Enrollments__c" extensions="ClassEnrollmentsExtensions">
    <apex:form >
        <apex:pageBlock >
            <apex:pageBlockSection Title="Class Enrollment" columns="1" id="classId">
                <apex:commandLink value="Add Class Enrollment" action="{!addClassEnrollments}" reRender="classId"/>
                <apex:variable value="{!0}" var="num"/>
                <apex:pageBlockTable value="{!ceRecord}" var="ce">
                    <apex:column headerValue="Youth Name">
                        <apex:inputfield value="{!ce.Youth_Name__c}"/>
                    </apex:column>
                    <apex:column headerValue="Works Wonders Record">
                    <apex:inputField value="{!ce.Works_Wonders_Youth_Name__c}"/>
                </apex:column>
                <apex:column headerValue="Attendance">
                    <apex:inputField value="{!ce.Attendance__c}"/>
                </apex:column>
                    <apex:column headerValue="Participation Note">
                        <apex:inputfield value="{!ce.Participation_Note__c}" required="false"/>
                    </apex:column>
                    <apex:column >
                        <apex:commandLink value="Remove" action="{!removeClassEnrollment}" reRender="classId">
                            <apex:param name="ceIndex" value="{!num}"/>
                        </apex:commandLink>
                        <apex:variable var="num" value="{!num+1}"/>
                    </apex:column>
                </apex:pageBlockTable>
            </apex:pageBlockSection>
            <apex:commandButton action="{!save}" value="Save"/>
            <apex:commandButton action="{!cancel}" value="Cancel"/>
        </apex:pageBlock>
    </apex:form>
</apex:page>


ClassEnrollmentsExtensions Controller:

public class ClassEnrollmentsExtensions {
    public Session__c sessionRecord{get; set;}
    public List<Class_Enrollments__c> ceRecord{get; set;}
    public Integer ceIndex {get; set;}
    public Id sessionId;
    public ClassEnrollmentsExtensions(ApexPages.StandardController controller){
        sessionRecord = new Session__c();
        sessionId = ApexPages.currentpage().getParameters().get('parentid');
        sessionRecord = [SELECT Id, Name FROM Session__c WHERE Id = :sessionId];
        ceRecord = new List<Class_Enrollments__c>();
        ceRecord.add(new Class_Enrollments__c(Session__c = sessionId));
    }
    public void addClassEnrollments() {
        ceRecord.add(new Class_Enrollments__c(Session__c = sessionId));
    }
    public void removeClassEnrollment() {
        ceIndex = Integer.valueOf(ApexPages.currentPage().getParameters().get('ceIndex'));
        ceRecord.remove(ceIndex);
    }
    public PageReference save() {
        upsert ceRecord;
        update sessionRecord;
        PageReference page = new PageReference('/' + sessionId);
        return page;
    }
    public PageReference cancel() {
        PageReference page = new PageReference('/' + sessionId);
        return page;
    }
}

Any help is appreciated!