What is the most popular gift for teachers?

Gift cards, by a mile. Coffee shop and Amazon cards come up most, followed by things that get used up, like good pens, treats and candles. Anything with an apple on it is fine, but most teachers already have a shelf of those.

When should you give a teacher a Thanksgiving gift?

The last school day before Thanksgiving break. It lands before the long weekend, and it doesn't get lost in the December pile of Christmas gifts.

Is it okay to give a teacher a Thanksgiving gift?

Yes, and it is usually a nice surprise since most families wait for Christmas. Keep it small, and check whether your school has a limit on gift value before you buy anything big.