从连接到键的哈希集中删除值?

我想让一个学生退学。以下方法没有删除该值?想法请:

public static void withdrawStudent(String student) {
    enrollments.values().remove(student);

}

public static void main(String[] args) {
        enroll("101", "Pat");
        withdrawStudent("Pat");
}

//这是课程的一部分。

private static HashMap<String, Set<String>> enrollments = new HashMap<String, Set<String>>();

public static void enroll(String unit, String student) { 
    Set<String> studentsSet = enrollments.get(unit); 
    if(studentsSet == null) { 
        studentsSet = new HashSet<>(); 
    } 
    studentsSet.add(student);
    enrollments.put(unit, studentsSet); 
}