Open with
Hi, it's Alex. I'm calling about today's class.
Then say
- I can't make the the 6pm class today.
- something has come up.
- Is there a make-up lesson?
- Could I get what I miss?
If they ask
- We need notice in advance.
- Understood. I'll tell you earlier next time.
- No make-up lessons.
- That's fine. Could I just have the materials?
- Will you be here next week?
- Yes, I can make next week.
Ending
Thanks for letting me know.