Because you already can't vape in schools, and it's been possible for boards to ban cellphones, but teachers don't have the backing of administration and they're certainly not protected from liability.
If I were a teacher knowing that if I were to bust little Johnny for TikToking in class or vaping in the washroom, I'd have to face a belligerent student and/or angry parents and the principal, superintendant and ministry would hang me out to dry, I wouldn't bother.
This is just another Ford government way to put the public sector in a lose-lose position so that they can, in this case, start out a voucher system run by private-sector buddies.