Probably should be moved to Legal Discussion...
First, not really familiar with Canadian laws.
Children also do not have the same rights as adults.
http://www.educator-resources.com/pdf/Teacher Tort Liability.pdf
Further,
In loco parentis, which means in the place of a parent, comes into play in a Teacher vs. Student situation. Consider that a 10 year old tells his friends that mommy came home drunk last night and the 10 year old gets grounded. Definitely not illegal, although probably bad parenting. This is not a perfect example and educators do not have ALL of the rights of a parent, but you get my drift. This principle "allows institutions such as colleges and schools to act in the best interests of the students as they see fit, although not allowing
what would be considered violations of the students' civil liberties." These liberties are severely curtailed in schools (in most jurisdictions) serving children (younger than age of majority).
http://en.wikipedia.org/wiki/In_loco_parentis
Further, if the teacher was terminated as a result and the student was the age of majority, the Twitter post could be considered libelous. But, again, children (younger than age of majority) do not and cannot have the full rights afforded an adult, especially in an educational setting.
The student's freedom of speech was not infringed. The definition of freedom of speech is to speak freely without censorship. The Twitter post was not blacked out by the school in conjunction with the suspension. The post was punished. We can argue the legal definition of censorship, but that would be arguing a moot point because again, children do not have the full rights of adults, especially in an educational setting.
Edit:
The word(s) "responsibilities/liabilities" could be used in place of "rights" in several instances in this post.