You don't get to choose. The CRA requires that the student claim the tuition amount they need in the tax year to reduce their tax payable to zero. Only what is left over after tax is reduced to zero can be carried forward to a future year.
In the next year, the same goes... if you had unused amounts from a prior year that carried forward, you must claim as much as is needed to reduce your taxable to zero... and so on.
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