On the Last day of May...

Things i didn't know how to respond to:

Student: "Ms. Daley, can you let me know when 20 minutes have gone by?"
Me: "Sure can."
20 mins. later
Me: "OK you have 30 minutes left."
Student: *angry* "Why didn't you tell me when there's 20 minutes left?"
Me: "I did ... it's a 50 minute class. You used 20 minutes. Now there's 30 minutes left."

B: "Ms Daley! Help! My butt cheeks are quivering!!"