[The Materials Below were Originally made for Middle School English Speaking & Writing Tests]
- my school uses the "Middle School English 3 (2019) by DongA
-So the grammatical points are based off of those covered in the text book.

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What about having students introduce their favorite food/ or comfort food?
They can introduce their favorite food or their comfort food (1 sentence), what's in in it and where it's originally from (2 sentences), explain a 3 ~ 5 step recipe (3 ~ 5 sentences),for how to make it, share a memory or reason behind why this is their comfort food, and end with a recommendation or hope for everyone to try it. 

If you can, I would suggest using one your classes for them to get prepared for the test, and you can give them a worksheet with the guiding questions in one of the classes and let them also use their smartphones or school tablets (if you schools have them) to do some research regarding the recipes.

I have created  a couple of writing / speaking test prep PPTs and Worksheets for both 2nd graders and 3rd graders at my middle school and the "comfort food" topic worked really well for the students!

I am attaching the PPT and Prep worksheets below as samples feel free to use them as they are or edit it to your needs.
For example, my students had to use the following grammar/ expressions below:
~ Have you ever + past participle
~ Not only ~~ but also ~~
~ I hope you can try it/ taste it/ ~ sometime
~ Noun make(s) + object (me/ us) feel + adjective (good/ happy/ excited)

but you can change those to suit the expressions your high school is expecting the students to learn ( if any)

Also, for middle school, we allow students to make a small hint card/ cue card that the can take a look at to help them remember what to say if they get too nervous or forget.

I wish you the best of luck in for the speaking tests! Those can be a bit tricky to plan.
I'm sorry I don't have any high school level materials as I've only worked at the middle schools here.

No worries!
I'm glad to be of some help to a fellow teacher (선생님) !

Hello, fellow high school teacher. I perform my speaking tests as follows: I meet with each student individually (in an empty classroom) and ask them a question. With this question I always ask a follow up question as well to get some details. I shuffle between 4-5 different questions to ask my students. An example would be “What’s a movie you’ve enjoyed recently? What was your favorite part? Why?”

I have approximately 30 students per class and must wrap up the exam by the end of each class period. Hope this helps you or someone else! ^^