In the last contract the union and the company announced the "Tower Progression Program".
The Tower Progression Program (TPP) is intended to improve qualification success rates for ATC Student Controllers at the Toronto Pearson Tower and the Winnipeg Tower where we have difficulty attracting, qualifying and retaining employees.
The short version is that in the YZ FIR staffing Pearson is the priority. In an attempt to increase qualification rates at Pearson, Ab Initio students will be assigned to a "feeder tower" where they will work until qualification with the stipulation that within 2 years following qualification they could get "called up" to Pearson. If you don't get called up in 2 years, you stay where you are. If you do get called up, and don't qualify at Pearson, you won't have return rights to your first unit, you'll get sent to another unit in the country.
In the Toronto FIR the "feeder towers" are identified as: CYKF, CYAM, CYXU, CYHM and CYOO.
Prior to the TPP VFR basic students were nearly always slated to go to Pearson. It's a pretty safe bet that students will go to one of the VFR towers listed above as part of the TPP.
Toronto Island is chronically understaffed as well, and it's plausible that people will get sent there outside of the TPP. YXU is also chronically understaffed.
Hope this helps, PM me if you want more specific information.