Assignments are meant to evaluate a students performance and a student is also graded based on the numbers that include numbers that he gets on those assignments. It is common in students that they do others assignments and vice versa. It is also common that students put their assigents as a task on online freelancing sites such as fiverr. I personally believe that both the person who does other's assignments and the one who gets those done by others are committing sin. However, I don't know if I can back my opinion with Quran and Sunnah. Plz guide me in this regard with regards to both the individuals respectively. JazakAllah Khair.
This is considered cheating and deception in Islam, they are forbidden and are characteristics of hypocrites. And both are heinous sins.
Your example scenario (rephrased):
If we suppose that X and Y are students, and Z is their teacher.
X is doing Y's work and assignments besides his work and assignments.
It is cheating since there's an act of dishonesty from X and Y's sides in order to gain a particular advantage.
It is deception because there's a plotting of an invisible, bad thing/act for causing damages and harm to the other party.
How are they applied to the scenario?
The cheating, in the scenario, appears as Y is letting X do their assignments instead of doing them himself. Both the cheater and the one who helped him cheating are considered cheaters.
The Prophet (Peace Be Upon Him) said:
Whoever bears arms against us is not one of us, and whoever cheats us is not one of us.
The deception, in the scenario, rises as Z believes (and this what appears to him) that the work of Y has been done by Y himself and thus the score and evaluation will be for Y based on his work.
In Arabic (the exact wording):
يُخَادِعُونَ اللَّهَ وَالَّذِينَ آمَنُوا وَمَا يَخْدَعُونَ إِلَّا أَنفُسَهُمْ وَمَا يَشْعُرُونَ
In English (in what it means):
They [think to] deceive Allah and those who believe, but they deceive not except themselves and perceive [it] not.
Of course, both X and Y are deceiving Z. So, what applies for Y of the ruling regarding deceiving is applied to X too.
And Allah knows best.
For reading further about cheating and deception, see here.