Canadian Payroll: how to suppress tax

Dear HCM experts,

We have students who will not earn more than their tax credits.  They have specified on the TD1 that their tax credits will be more than their earnings and they request the employer not to withhold tax (page 2 of the Federal TD1 "Total income less than total claim amount").  I know that we can suppress tax by entering '999999' in the employee's tax credits but I need a way to suppress tax while properly recording the employee's tax credits as indicated on the TD1.   It is not possible to make earnings codes tax exempt as other employees using that