In applying for BTO, HDB looks at your income via your CPF statements. It doesn't matter whether or not you're a full-time student. Rather, it's what your declared income (and hence CPF-liable income) is. As for grants, even if your income exceeds a certain bracket for the grant, you may still qualify for a lower bracket and be disbursed a lower grant amount. Hope this helps and feel free to drop any more questions!