The head's on a course, the deputy's in a traffic jam. Nobody else is available. If you were asked to take assembly in 20 minutes' time, with no previous experience, could you do it? Would you step forward and volunteer?
Despite any anxieties you might have, you should accept the challenge. It will bring you big brownie points, and it's good experience.
Start by freeing up as much of those 20 minutes as you can. Swap your duty and get a colleague to bring your class in. See if you can get somebody to cover registration.
At the same time, make sure someone's going to be in assembly with you. In some schools, staff sometimes stay out of assembly. Not today.
Begin by working with what you have. If someone usually plays a hymn, then ask them to choose one and quickly read it to see if you can base the assembly on its message. Hymns and songs are often disconnected from the rest of the assembly. By making the link you're doing something positive.
You can sing it again after you've told the children what it's about.
Check with colleagues to see if there's anything to give out, such as swimming certificates and merit awards.
If so, consider making this the meat of the assembly rather than an add-on.
As you give each child their award, ask them questions, draw them out and praise them.
Children are accustomed to the one-to-one sports interviews on television and you'll find some will really rise to the occasion and give you a good "double-act".
Finally, pull it all together with a reflection on the qualities needed for success.
Ask colleagues if their pupils have brought anything from home for a lesson or to show. This could include items such as family photos or vintage household gadgets.
Base your assembly on some of these, doing a dialogue with the teacher concerned: "You're doing this in class aren't you Mr Smith? Can you tell us about it?"
You could also read or tell a story. Use an assembly resource or a passage or poem from a popular book. Draw out a moral if you can, but don't worry about this too much.
If you make up a story of your own, keep it simple. Children like anything at all about your own life - your childhood, pets or children.
Follow the school custom on having a prayer or reflection time at the end of assembly.
Don't be tempted by technology. PowerPoint, whiteboards and data projectors are wonderful, but they require preparation.
When the time arrives, get into the hall before the children - take your own class in early if necessary. Look confident and in charge as the students come in. Smile and make eye contact with lots of children.
As you talk, keep looking around. Fix on anyone who's being inattentive - catch their eye, raise your eyebrows and pause meaningfully if necessary.
Try not to interrupt your flow by telling students off.
Keep the whole thing brief, pacy and simple. Resist rambling because you don't know how to finish. Just stop.
Then at the end of assembly, smile and say a simple "thank you" to the children and staff:
"Thank you, boys and girls. Thank you, teachers. Have a good day."
Another job well done.
Gerald Haigh is editor and lead writer of Primary Assembly File from PfP Publishing www.pfp-publishing.com