How to say farewell to a teacher the end of senior year?

I know what you mean... One of my teachers was like that to me. I cried and cried so much thinking I'd have to say goodbye to her, it was tearing me apart! But anyway, I got her a gift and wrote her a really long letter telling her everything, I didn't hold back. I let her know just how much I appreciated everything, how I didn't want to say goodbye and how she was like a mother to me. I wrote my phone number in it too, and told her if she ever felt like catching up, we could keep up our coffee tradition (she used to bring me up to the staffroom when I didn't have class and we'd have coffee and a chat regularly). I gave it to her on my last day (very emotional day...) and I got a call the next day. We've stayed close ever since, she didn't wanna say goodbye either. :) So try it :)