如何在保存之前验证ManyToManyField？

Question

我有类似的模型：

School:
    id
    name

Teacher:
    id:
    name:
    school = ForeignKey(School)

Student:
    id:
    name:
    school = ForeignKey(School)
    teacher = ForeignKey(Teacher)

当用户从表单创建Student对象时，我需要验证学生和教师是否拥有相同的学校，我该怎么做？