Here is a suggested pattern for your letter:

Thank you.

What you liked about the talk.

What you learnt.

Something about you.

Something about the class.

Conclusion.

23rd May, 2009

Dear Mr Smith.                                                                                          

Thank you very much for speaking to our class.

I greatly enjoyed your talk about how you love making paper planes. I was fascinated how you made them and how people have bought so many of your books.

I learnt many things from your talk. Especially how hard you need to work and how learning is for life.

My passion is growing flowers and one day I would like to be a florist. At the moment I have the opportunity to be mentored by Mrs Rose at Garden Flowers. I am really enjoying the experience and learning heaps about the workplace and serving customers.

Our class at the moment is busy with our individual projects and I am planning our outdoor camp which will be skiing at Mt Hotham. I never realised how much work is involved.

All the best for the future.

Yours Sincerely,

(signature)

John Student