Rabbi Fisher’s students looked up, as the classroom door swung open. Expecting the stern countenance of Rabbi Fisher, the students were pleasantly surprised to see… a substitute! The teacher introduced himself as Rabbi Cohen, and, much to the boys’ chagrin, began the lesson.
Several of the boys took the opportunity of their regular teacher’s absence, to discuss the soccer tournament which was to take place during the next recess. Rabbi Cohen raised his eyes, and his gaze settled on Yossi, who was sitting amidst the group of offenders. In truth, Yossi hadn’t spoken, but Rabbi Cohen didn’t recognize the boys, yet, and mistakenly took Yossi for one of the talkers.
"You!" Rabbi Cohen extended his finger in Yossi’s direction. "Come see me after class."
Yossi proclaimed his innocence, but his protests only further enraged the teacher. "Look at all the disruption you’re causing! You’re clearly not paying attention to the words of Shirat Chana that we’re learning, now. As a result, I’m requiring you to memorize all of Shirat Chana tonight, and come back to me tomorrow to be tested!"
Yossi, seeing that there was nothing he could do, sat down quietly and waited for the lesson to be over.
The next day, right on schedule, a student sauntered into Rabbi Cohen’s classroom. The boy recited Shirat Chana by heart, flawlessly. Rabbi Cohen complimented him, both on how responsible he was in fulfilling his assignment, and on his accuracy. The boy smiled and nodded, then ran outside, and burst out laughing.
"How’d it go?" Yossi asked Gili, his identical twin brother.
"Amazing! Rabbi Cohen even complimented you on how well you knew the pesukim."
"Thanks for filling in for me!" Yossi enthused, as he gave Gili a slap on the back. "Not everyone has an identical twin who enjoys memorizing pesukim in Navi."
Was Yossi allowed to have Gili be tested instead of him?

Anwer of Rabbi Avigdor Nebenzal:
Ordinarily, it would be forbidden to fool a teacher, as Yossi and Gili did. However, because Yossi was punished unjustly, in this case, he was allowed to have Gili be tested instead of him. However, if Yossi were to receive some kind of benefit, such as a prize, for doing well on the test, it would be forbidden for him to have Gili be tested instead of him.
Summary: Because Yossi was only trying to save himself from unjust punishment, what Yossi and Gili did was permitted.