Why not have the kids come up with a list of what they've learned this year, and have them help you write the letter telling about all they have done and learned?
Another suggestion is to write individual letters and brag about all the things their child has accomplished this year. This will probably take a bit longer than you expected to spend on it, but it's the kind of thing that they will LOVE. What parent doens't like to have their child bragged about, especially when they don't have to do it themselves?
I would send a letter or email that list the children and one things about each child. The parents like to see this about their own child but also about their child's classmates. For example - Bob was very helpful this year, Emily was a great encourager to her classmates, Jill was very creative in all her projects, etc. Hope this helps you end the year with a positive thought about each child.
Today I give you back your child. The same child you confidently entrusted to my care last fall. I give them back pounds heavier, inches taller, and months wiser than they were then.
Although they would have attained their growth in spite of me, it has been my pleasure and privilege to watch their personality unfold day by day and marvel at this miracle of development.
I give them back reluctantly. Having spent ten months together in the narrow confines of a classroom, we have grown close and we shall always retain a little of each other. We have lived, loved, laughed, learned, and enriched our lives together this year.
Remember that I shall always be interested in your child and their future endeavors, wherever they go, whatever they do, whoever they become. Thank you for allowing me the opportunity to be your child’s teacher this year!