That's such a terrible strategy...

Designate a white mage and give white magic to them only, then designate a black mage and black magic to them only.

The reason your spells "go away" after you use them in battle is because you're using them as an item. This causes the spell to be cast at level 8, which is somewhat equivalent to fira/thundara/blizzara in other games. Use ful if you have extra copies early on in the game and you're fighting a boss with a weakness to that element, otherwise it's not so good. Usually you're better off just selling your extra spells anyway.

Magic in FFII is kind of a tricky affair. PM me for extensive help on magic (and party setup in general) as I'd rather not eat up bandwidth by posting the full version on the boards.