Cast a ward and hold it up, facing Tolfdir. He'll cast one firebolt and quest complete, talk to him
And if you only have 100 magicka at the time, I'd recommend eating a bowl of Elsweyr Fondue to significantly boost your magicka. To get the materials for that, look in the invisible Dawnstar caravan chest for moon sugar, and in taverns for ale and eidar cheese wheels. You can also find cheese in the Riverwood Trader.
You'll have to maintain the ward for a second, he doesn't instantly cast the spell. You'll also have to be facing him.