What is the ruling on buying subsidized bread for another person who then uses it to cut in line unfairly, and is eating it considered unlawful, and is the doer sinful?

It is not permissible to transgress upon the right of another by cutting in line ahead of them. A person who is behind taking the share of bread belonging to someone ahead of them is a forbidden transgression according to Islamic law. You must explain to the student that your refusal is not an unwillingness to serve them, but rather an aversion to transgressing upon the right of another. If all those ahead of you seek permission and grant it, then it is permissible for you to take their share. The common practice of this action does not make it permissible, due to the lack of verification of the consent of all those ahead in line. Seek forgiveness from Allah for what has passed and do not return to it. The purchased bread is lawful to eat, and the sin falls upon the one who cut in line ahead of the rightful owner without their permission. If known, it necessitates seeking their forgiveness; otherwise, repentance, seeking forgiveness, and not returning to the act are sufficient.