First of all it is not a strike because it is not part of the collective bargaining process because with a government imposed contract, imposed under a draconian law whose constitutionality is under challenge, there is no collective bargaining process. Secondly the political protest is aimed at the government as policy maker and legislator, not as employer.
It is not illegal because political protests are not illegal in Canada, even if Stephen Harper and Dalton McGuinty wish they were.
Not that the teachers are completely free of consequences. For one they will not be paid for the day, and secondly their contract provisions may provide for discipline for absences not approved by the employer. But any such action will be subject to the grievance and appeal provisions of their contracts. That being said the usual discipline by employers for such absences is suspension from work without pay. So theoretically the school boards could force the teachers to take time off work without pay as punishment for taking time off work without pay. Sort of like suspending students for truancy.
The biggest irony, or perhaps more appropriately described as hypocrisy, is that Dalton McGuinty has his knickers in a knot over teachers taking a day off without pay to protest a process that has imposed a contract on them that requires them to take days off without pay.