<apex:repeat value="{!mycourses}" var="course">
    <apex:outputText value="{!PercentComplete}">
        <apex:param name="courseIdDetail" value="{!course.id}" assignTo="{!courseIdDetail}"/>
    </apex:outputText>
</apex:repeat>

public String courseIdDetail {get;set;}
public String getPercentComplete(){
    System.debug('courseIdDetail from page = ' + courseIdDetail); //coming back as null
    AggregateResult[] avgProgress = [select AVG(Progress__c)avg
            FROM Tests__c
            WHERE Course__c =: courseId }
    String progress = String.valueOf(avgProgress[0].get('avg'));
    return progress;
}

public class CourseWrapper {
    public Course__c course {get; set;}
    public CourseWrapper(Course__c c) {
        course = c;
    }
    public String getPercentComplete() {
        ... WHERE Course__c =: course.Id ...
    }
}

public List<CourseWrapper> mycourses {get; set;}

<apex:repeat ...>
    <apex:outputText value="{!course.percentComplete}" />
</apex:repeat>