Forgot to mention I would likely not ever rent to students unless grad student and obviously not a party hound.
This would likely be a violation of Fair Housing Laws, unless you have a set, objective formula to determine what a party animal is. Renting to only Grad students would be another major violation.
I am not a lawyer, real estate agent, or landlord but I'm pretty sure this is not true. Under the fair housing laws, you are not allowed to discriminate against people based on the following protected statuses: race, color, religion, national origin, sex, disability, and familial status (i.e. whether they have kids, or are pregnant, or are planning on having kids). In fact, in the city of Boston, there is a city law that says you can't have more than 4 unrelated undergraduate students in a rental unit. You absolutely can discriminate against people for reasons unrelated to their protected status - say "I will not rent to anyone who plays volleyball!" or "ABSOLUTELY NO MINNESOTANS ALLOWED!" or "NO UNDERGRADUATES."
Now, if by "only renting to grad students" you mean "not people with kids" that would be a violation.
Also, it's pretty easy to set an objective formula that screens out the vast majority of party-animal undergraduates: require a certain income level and don't allow cosigners.
*Just noticed OP is in Canada - where, maybe with variations by province, "a landlord cannot discriminate in tenancies based on a person’s race, colour, ancestry, place of origin, religion, marital status, family status, physical or mental disability, gender, sexual orientation, age or legal source of income." So maaaaaybe you could get in trouble under "age" or "legal source of income." And my comments about banning Minnesotans would probably be illegal in Canada, since that would be place of origin.