What's happening is when you first entered your slips, your income was low enough that you were assigned the Working Income Tax Benefit. (WITB). When you added your Tuition Slip, the WITB was removed from your tax return as students are excluded from this benefit.
It can be tempting to remove your tuition to receive that extra benefit but it's not a good idea. CRA's matching program will eventually discover the missing tuition slip and you'll be required to repay the benefit plus interest. It's just not worth it.