Hello Sara75,
As PMM stated, English as a Second Language students are not eligible to work-off campus. It seems that Service Canada made a mistake by giving you a SIN #.
please read the last paragraph below from the Canada Gazette -Regulations Amending the Immigration and Refugee Protection Regulations
Regulatory amendments to work permit programs
The Regulations also allow full-time students pursuing an academic, professional or vocational training program at a designated learning institution to work off-campus without a work permit. Off-campus work is allowed for up to 20 hours per week during the regular academic session and full time during regularly scheduled academic breaks. This represents time and cost savings to students who no longer need to apply and pay for a separate work permit and for CIC, as officers will no longer process off-campus work permit applications. This amendment replaces the existing program design, which requires study permit holders to wait six months after beginning their program of study before applying separately for an off-campus work permit.
The effects of these amendments will be to exclude those students undertaking English or French as a second language (ESL/FSL) programs, or general interest or preparatory courses from the authorization to work under the off-campus work permit exemption or under the co-op work permit program.