"I provided myself with various musical instruments… it was clear that it was all futile" (Kohelet 2:8-11)

The students had arrived earlier that day, bearing expressions of confidence, seriousness and excitement. They filed into their dorm rooms, and began unpacking freshly washed clothing, delicately scented with fabric softener and wistful prayers for a good school year.
That evening, the entire student body gathered together, for the first assembly of the year. The principal went through the usual welcome, review of policies, procedures, and other miscellany. Before the meeting ended, he added one final note.
"My dear students, you know how much your teachers and I care about your wellbeing. This yeshiva is a place of holiness, where we try to share with you lessons of sanctity and wholesomeness. It goes without saying that music or literature of a secular, vulgar nature, which runs contrary to everything that our school stands for, will not be allowed anywhere on the school premises, including the dorm rooms. If such material is found, the literature or radio will be confiscated."
The students stood up from their seats, and headed back to their rooms. Some of the teachers remained behind, in the staff room. One teacher began speaking. "I’m not sure about the last thing that the principal said. Of course he is entirely right, that such materials don’t belong in our school. I’m just wondering about the implementation. If I’m not mistaken, the parents were never notified about this policy, that their sons’ belongings would be taken, if they listen to in music that’s not in the spirit of the school. It seems, then, that taking those items would be stealing!"
Another teacher spoke up. "All of the parents know where they are sending their sons. They know the values of our school. Do you think that parents who send their children here would want their sons listening to such music? It’s clear that the parents would want us to implement this policy, and it’s not theft at all!"
The discussion continued, with different teachers voicing their opinions, until the principal walked into the room. "My dear colleagues, I hear that there is a debate taking place, as to whether or not we are allowed to confiscate radios from our students. The truth is that I did not notify the parents regarding this policy, because I didn’t want to start any issues before the school year even began. I felt that the best way to handle the situation was to notify the students of this policy directly, on the first day of the school year. I do not believe that there is any question of theft, regarding this matter." With that, the impromptu staff meeting concluded, and the teachers went home.

One day, one teacher, Rabbi Shlomo, was walking in the courtyard, during the midday break. As he neared the dorm, he couldn’t help but hear raucous, pulsating, earsplitting music, emanating from one of the dorm windows. He cautiously entered the building, and followed after the blasting sonic experience.
It didn’t take long to identify the source of the music, as the door to the room seemed about to lurch off its hinges, as a result of the vibrations coming from behind it. Rabbi Shlomo swiftly turned the handle, to see five boys in mid boogie, disconcertedly staring back at him.
Rabbi Shlomo turned off the radio, unplugged it from the outlet, and tucked it under his arm. After announcing that he expected to see the boys in his class room, in half an hour, he left the room.
Half an hour later, the boys nervously filed into the room, as instructed. Rabbi Shlomo patiently, but firmly, explained why their actions were inappropriate, and reminded them of the warning issued by the principal on the first day of the school year. The boys nodded silently, and left the room with their heads hanging.
Rabbi Shlomo was left in a quandary. Was he, in fact, permitted to take the radio, or did it constitute theft?

Answer:
Rabbi Moshe Feinstein, zt"l:
The Gemara (Yevamot 89b) states that a bet din (Jewish court of law) may cause an individual to lose rights to his property. This is evident from the verse in Ezra (10:8), which says "And whoever does not come by three days, according to the counsel of the chiefs and elders, all his property will be confiscated…" However, that verse refers to a decree affecting all of the Jewish people.
We also find that leaders of an entire city can enact rules which affect its citizens. Perhaps there is reason to say that a yeshiva has the status of a city, in some fashion, and can enact rules affecting the student body. However, it is not clear that the cases are parallel.

Given the uncertainty, it is difficult to permit the school staff taking the radios. This is a situation involving possible theft, which is a serious crime. Additionally, the radios might be the property of the parents, and not the students.
In practice, the radios should be kept until the end of the school year, and then returned to the students. Regarding reading material with forbidden content (e.g. immorality, and philosophy contrary to the Torah), perhaps it is permitted for the school administration to burn the books. This would seem to not be theft, because the administration would be acting to protect their students from sin.
(Masoret Moshe, section 1, Laws of Theft)