You do not have to claim your tuition but at the same time you cannot claim the Canada Workers Benefit as a full-time student. You will benefit from claiming your tuition when it is used to reduce your taxes in future years when your income is higher.
What is happening is your tax return was originally showing a refund of the Canada Workers Benefit (CWB) that full-time students are not eligible for. As soon as you enter your T2202A, the credit is removed and your refund is reduced.
Based on your employment earnings the software calculates your entitlement to the CWB, however, the government disallows this benefit to students. So entering the T2202 Tuition Certificate slip causes a recalculation and removal of this amount from the refund.