in my opinion, the B-F bond is described as covalent for simplification (and convenience) in high school chemistry syllabus. i don't think a high school student is expected to understand it to such rigourous level.
With reference to the attached MO diagram, all MO energy levels are filled with electron pairs, except the two highest ones. there are 6 pairs of electrons not involved in bonding (see orbitals 1a2', 1e'', 2e' and 3a1'). There are 12pairs of valence electrons in all, meaning 6pairs are involved in making a molecular orbital. This means each F contribute 2 pairs of electron. However, to not actually violate octet rule, it means each F atom contribute one electron in making a B-F bond, and contribute an electron pair partially into boron orbitals.