EI premiums were deducted from a student's pay cheques in 2021. Since they cannot claim EI, can this be entered on line 45000 as an overpayment of EI or otherwise rebated

There is no age limit for deducting EI premiums, and there is no age limit on who can collect EI.  So there is no way of having the amount refunded, unless it was an overpayment.