It is first period.
"Good morning, children," said their math teacher, cheerfully,
"Because of how proud I am of your scores last week, I've made my favorite sixth-grade prodigies some marshmallow squares! Now, I want you all to finish your tests, and when everyone is done, you all get your treats!"
The sixth-graders, having had her marshmallow treats before, were excited to try another batch. They all rushed and finished their tests, with exactly 31 minutes left in class. The teacher passed out the marshmallow squares, and with delight, they devoured them quickly.
The first reaction to the poison triggered one minute, twelve seconds after consumption. One by one, as if by a domino effect, each student fell under the influence of the poison. They retched violently, eyes watering, but the poison's effects continued.
They soon lost feeling in their feet and fingers, lost control of their facial muscles, and lost their hearing. Their eyesight went soon after, their throats practically sealed shut, and the numbness spread to their arms and legs. One by one, they collapsed into their vomit, twitching, with no control of any of their muscles. 3 minutes, 17 seconds. They were all dead.
She wiped up their sick and collected the bodies into garbage bags. She took them to the dumpster outside, and went into her classroom again with air fresheners. She sprayed the cheap aerosol scent around the classroom, smothering the scent of death with Vanilla Lavender. 23 minutes, 2 seconds.
She had almost exactly 3 minutes and 30 seconds until her Second Period class came in. She changed out of her vomit-smeared shirt, sprayed it with air freshener, and hid it under her desk. She sanitized her hands and waited for the next class.
It is second period.
"Hello, children! Seeing last week's test scores, I must say, I'm proud of you all. For after your test, I've made you all marshmallow squares.."