Hello!
The correct answer is: We students are participating in a meeting at the end of the school day.
An easy way to tell if a sentences uses "we" or "us" is to remove the subject (students, in this sentence) and see which word works correctly.
In this case, it would either be "we are participating in a meeting at the end of the school day." Or "Us are participating in a meeting at the end of the school day."
By doing this, you are able to see which word works correctly with the sentence.
I hope this helps answer your question! Have a great day!