The Occupant Load Ratings are calculated by the Ministry of Municipal Affairs and Housing for the Province of Ontario using a number of different calculations and are found in the Ontario Building Code.
Once the numbers have been calculated, the only way they can be revised is through renovation and/or construction (change style of seating etc.).
If a lecturer were to increase the size of his/her class by letting the students bring extra chairs into the room or by letting the students sit in the aisles, they could conceivably be charged by the "Authority Having Jurisdiction" (Fire Department), if someone were to be injured, or worse, due to overcrowding during an emergency situation.
The following are excerpts from The Ontario Fire Code:Section 2.7