tia, I have done a similar thing a few times in my life and find it to be wonderfully rewarding. I sat down when I had finally finished my education and evaluated the impact that my teachers had had on me. In the final analysis, only a handful had been truly life changing to me. I wrote 5 letters to these individuals, who had been teachers over a span of 28 years, (I started to school in 1954, and didn't finish my education until 1982) and just told them how much of an impact they had made on my life. All were delighted to hear such praise. I also sent a similar letter to a boss who had fired me from a job. He had been completely correct in doing so, and I was able to draw from that experience as well. He was most surprised to get that letter!
These kind of things are easy to do, cost nothing, and not only does the recipient of such a letter get great benefit, but the sender gets a great feeling as well.