As some of you commuters may already know, the process by which you get your GO ID, to be eligible for the student discount, has changed. Compass is no longer allowed to print the cards, so here is the process you must take to get your new one. (As taken from http://www.mcmaster.ca/sustainability/at_transit.html)
The earlier you get it done, the better. If you already have a card check the expiry date, because the website says that the card "expires October 31 of the following year".
Good luck 
GO Transit ID Cards
GO Transit offers student discounts of approximately 10% to 30% on the
adult 10-ride ticket price and
adult monthly passes with a valid GO Student ID card. You can't get a student discount without this card. Did we mention that the card is free?
You are eligible for a GO Student I.D. card if:
You are fully-registered and meet minimum academic load requirements established by GO Transit.
Undergraduate students require registration in at least 9 units.
Graduate students must be fully registered. For students completing their academic term in the spring, the card is valid for the summer period as well. For students applying solely for the summer study period, you must be registered in at least 6 units in both summer terms to get a GO ID card (i.e. 12 units min.).
APPLICATION PROCESS:
STEP ONE: Obtain a letter stating that you are a full-time student and your session start and end dates. Note: obtaining this letter may take a few days depending on how busy the office is.
Where to obtain this letter:
Undergraduate Students: Office of the Registrar
Graduate Students: Graduate Studies Office
Medical Students: Education Services Office, MDCL 3101 W12
IAHS Students (full-time): Mohawk Registration Centre IAHS Rm 121
Divinity students (full-time): Divinigy College, Business Ofice Rm 211
MBA (co-op): DSB Rm 112
MBA (full-time): DSB Rm 104
Bring this letter to get a special form authorized by a University representative (they have the forms, so don't worry about finding them ahead of time). Here's where you go to get your GO ID form authorized on campus:
Undergrads: Parking Office, E.T. Clarke Centre
Grad students: Parking Office, E.T. Clarke Centre
MBA (co-op): DSB Rm 112
MBA (full-time): DSB Rm 104
Divinity students: Rm 211, Business Office, Divinity College
IAHS students (full-time): Mohawk Registration Centre IAHS Rm 121
Med students: Education Services Office, MDCL 3101 W12
STEP TWO: Deliver the signed form to Union Station. This can be done in one of three ways:
1) Take the signed form to Union Station Customer Service where your card will be made up on the spot. Hours are weekdays 7 a.m. to 8 p.m., weekends and statutory holidays 10:20 a.m. to 6 p.m.
2) Take the signed form to any GO Train station. It will be sent to Union Station and processed, and then returned to the station of your choice for you to pick up. (Please allow 2-4 weeks for processing).
3) Mail the signed form, along with a self-addressed, stamped return envelope to: Union Station, Customer Service, 140 Bay St., Toronto, M5J 2L5. (Please allow 2-4 weeks for processing).
For students registered for 7 consecutive months or more, GO Transit ID cards are valid throughout the summer months and expire October 31 of the following year. Your student 10-ride ticket does not expire, but it can only be used when you have a valid GO Student ID card, which may mean holding onto any leftover GO student tickets until you get your GO ID card for the next school year.
