“A person who helps you solve problems you’d never have without them.” This teacher definition has clearly spent some time in a classroom. Set out like a dictionary entry in crisp black type, it’s a card for the teacher who can take a little cheek along with a genuine thank-you.
Send it at the end of term, for a teacher’s birthday or after they’ve explained the same thing for the third time without visibly losing hope. Add your own appreciation inside. No need to show your working.