I suspect you had enough income to qualify for the Working Income Tax Benefit, which is a refundable credit.  However full time students are not eligible for this credit.  If you were in full time education you need to file accordingly, as CRA does match up slips later in the year, and will reassess you if necessary. 

http://www.cra-arc.gc.ca/bnfts/wtb/fq_qlfyng-eng.html#q2