I couldn't find their complete policy on their website, but based on the superintendent's statement in the above link, it appears to be rather general and all-encompassing, without spelling out each particular violation specifically.
While my original position on this issue was that the school should have the legal right to do as they wished in this case, but that their actions were wrong in a moral sense altho legitimate in a legal one, after reading the school's mission statement....
Ontario Christian School
... is a community of believers sharing a common faith and commitment to:
A Sovereign God
The Lordship of Jesus Christ
The Power of the Holy Spirit
And the infallible, inerrant Word of God
Furthermore, compelled by our life-time responsibility to be salt and light in this world, we dedicate ourselves to imaging God to our children.
Our Purpose
... is to equip students to view themselves as transformers of society, using their unique personalities and talents to joyfully serve Christ and others in this world.
Mission Responsibility
As PARENTS AND A BROADER CHRISITAN COMMUNITY, we will...
Support the home and church in teaching Biblical Values
Nurture children and care about their futures.
Seek to train every generation to be obedient to Christ and to claim every area of life and learning for Him.
Commit ourselves to support teachers and staff as extensions of the parent's authority.
Strive to provide the resources and facilities necessary to carry out the mission of the school.
Strive to provide an education that develops each student's unique physical, intellectual, emotional, social and spiritual potential.
Celebrate our oneness in Christ by providing opportunities to support and serve.
In regard to our STUDENTS, we will...
Encourage them to develop a personal relationship with Jesus Christ.
Expect them to exhibit a lifestyle consistent with our calling to be imitators of God.
Provide and consistently enforce a behavioral standard that we believe is pleasing to our Lord.
Attempt to maintain a school atmosphere that will instill in our students a spirit of enthusiasm and joy in the Lord.
Our PROGRAM will...
Provide a course of study that is grounded in Biblical truth and that leads students to see God's immeasurable greatness, awesome workmanship, and boundless wisdom.
Enable students to integrate Christ into every aspect of their lives.
Prepare the students to carry God's message to a broken and sinful world.
As a FACULTY AND STAFF we will...
Demonstrate our Christian faith by a personal walk with the Savior, a reliance on the power of prayer, a commitment to knowing God's Word, a life of service, and church involvement.
Encourage each student to apply the Christian faith to both learning and life and, thus, develop a Christian world-and-life view.
Commit to maintaining professional competence, updating teaching skills and completing continuing education.
Support one another in our united effort to fulfill our mission.
Based on the above, I have to change my view somewhat.
Seems to me that since this school feels so incredibly strongly about their mission, their role, etc., that you can't really argue with them for expelling this student for the reasons which they did.