Address the following:
- what's going on here at the end of the school year--fun activities, Prom, finals, graduation, etc.--and what you've been up to, how you're feeling
- explain that the semester/our class is almost over and that you'll be graduating or moving on to other classes
- share where you're headed (summer plans, college plans, travel plans, etc.)
- either say you will try to keep in touch or explain that you won't be writing anymore
- wish him or her well (have a great summer, keep writing, etc.)