CEGEP teachers are part of the public sector, thus may fall under the same guidelines as high school teachers. CEGEP teachers are part of a similar union environment and collective agreement.
The eligibility rules for the Eligible Educators School Supply Tax Credit doesn't specifically exclude CEGEP teachers.
If you feel like you could easily defend the claim you make if challenged by the CRA, consider making the claim. Note that the CRA states that they may demand your employer provide certification that the supplies were required. If you don't think the school would do that, it might be best to omit the claim from your return.