Half your teachers’ names will slip away by thirty. The one who stayed after class to explain fractions again won’t.

What survives is not the quantity of teaching but the moment somebody chose to give you extra. Effort offered freely, on their own time, registers differently and keeps registering for decades afterwards. Everything else blurs into the general background of having been taught.

If one of those people is still reachable, tell them. It takes a paragraph and they almost never hear it. And when you get the chance to be the one who stays behind for somebody, take it, because that is the part that lands.