Well, I did it. For science.
Had 199% skill exp and hit Magic level 2 after a skirmish.
However, spells that fail to do damage ("Your magical attack was useless!") apparently
give any magic skill experience. Casting low level magic on a 200 year old vampire didn't make my magic skill percent budge even a bit.
To be fair, I don't think competent players that have specific builds planned for their characters are likely to start training magic 21 levels after creating a character.
I was fortunate to run into a weak monster. Casting level 2 magic on a mammutfly larva did grant experience and raised my magic level to 3. Looks like I'll have to seek out low level monsters until my spells can actually harm more level-appropriate ones.