The law makes it an offence to organize or partake in a protest "that could result in" a student being blocked from a classroom, where the demonstration is in or within 50 metres of a school building. It is also an offence to "contribute to slowing down, degrading or delaying" university or college classes.
Violators face fines of $1,000 to $5,000 for individuals and up to $125,000 for a student or faculty association. The amounts double for a subsequent offence.
The measure was passed in May as the Liberal provincial government sought to contain the sometimes tumultuous protests stemming from Quebec's student crisis.
The Montreal Police Service said in a communiqué Friday evening that it won't take action on the provisions relating to demonstrations blocking or delaying access to schools, or others forcing professors to report to work, unless a school's administration makes a request for police intervention.
Spokesperson Cmdr. Ian Lafrenière said it wouldn't make sense to disperse protesters and free up an entry into a school building if, for example, classes were already cancelled.
"Imagine for a moment we force that entry, we get two platoons there to open a barricade to force the entry, and once we get in, we figure out that there's no classes. So that's why, if we do an intervention, it will be in co-ordination with the school authorities," he said.
"I know it might look stupid, but it's so basic: If you're using force to get people in, and in the end there's no people taking classes, that's not very helpful."
The police force said it will still enforce the Criminal Code, which addresses offences like unlawful public assemblies.
Bill 78, now also known as Law 12, has triggered widespread demonstrations across the province by people who didn't necessarily support the student cause, but were concerned by the legislation's restrictions on the right to protest.
Parents with their children, seniors, students, advocacy groups and labour federations have gathered in "casserole" protests where they bang pots and pans while marching through the streets.
The law has a sunset clause and expires next July 1.
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